fix for nonunique index checking JSON serialization

[shouples]

Adds tests and handles an issue where checking if a series is json serializable can raise a ValueError if the index values aren't unique, due to the underlying pandas json writer.

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Input In [1], in <cell line: 5>()
      2 british_youtube = pd.read_csv("GBvideos.csv")
      4 df = pd.concat([canadian_youtube, british_youtube])
----> 5 df

File /opt/conda/lib/python3.9/site-packages/IPython/core/displayhook.py:262, in DisplayHook.__call__(self, result)
    260 self.start_displayhook()
    261 self.write_output_prompt()
--> 262 format_dict, md_dict = self.compute_format_data(result)
    263 self.update_user_ns(result)
    264 self.fill_exec_result(result)

File /opt/conda/lib/python3.9/site-packages/IPython/core/displayhook.py:151, in DisplayHook.compute_format_data(self, result)
    121 def compute_format_data(self, result):
    122     """Compute format data of the object to be displayed.
    123 
    124     The format data is a generalization of the :func:`repr` of an object.
   (...)
    149 
    150     """
--> 151     return self.shell.display_formatter.format(result)

File /opt/conda/lib/python3.9/site-packages/dx/formatters/dataresource.py:127, in DXDataResourceDisplayFormatter.format(self, obj, **kwargs)
    124 def format(self, obj, **kwargs):
    126     if isinstance(obj, tuple(settings.RENDERABLE_OBJECTS)):
--> 127         handle_dataresource_format(obj)
    128         return ({}, {})
    130     return DEFAULT_IPYTHON_DISPLAY_FORMATTER.format(obj, **kwargs)

File /opt/conda/lib/python3.9/site-packages/dx/formatters/dataresource.py:80, in handle_dataresource_format(obj, ipython_shell)
     78 orig_obj = obj.copy()
     79 orig_dtypes = orig_obj.dtypes.to_dict()
---> 80 obj = normalize_index_and_columns(obj)
     81 obj_hash = generate_df_hash(obj)
     82 update_existing_display = obj_hash in SUBSET_TO_DATAFRAME_HASH

File /opt/conda/lib/python3.9/site-packages/dx/utils/formatting.py:55, in normalize_index_and_columns(df)
     52 display_df = df.copy()
     54 display_df = normalize_index(display_df)
---> 55 display_df = normalize_columns(display_df)
     57 # build_table_schema() doesn't like pd.NAs
     58 display_df.fillna(np.nan, inplace=True)

File /opt/conda/lib/python3.9/site-packages/dx/utils/formatting.py:112, in normalize_columns(df)
    110 logger.debug("-- cleaning before display --")
    111 for column in df.columns:
--> 112     df[column] = clean_column_values_for_display(df[column])
    114 return df

File /opt/conda/lib/python3.9/site-packages/dx/utils/formatting.py:142, in clean_column_values_for_display(s)
    139 s = datatypes.handle_complex_number_series(s)
    141 s = geometry.handle_geometry_series(s)
--> 142 s = datatypes.handle_unk_type_series(s)
    143 return s

File /opt/conda/lib/python3.9/site-packages/dx/utils/datatypes.py:194, in handle_unk_type_series(s)
    193 def handle_unk_type_series(s: pd.Series) -> pd.Series:
--> 194     if not is_json_serializable(s):
    195         logger.debug(f"series `{s.name}` has non-JSON-serializable types; converting to string")
    196         s = s.astype(str)

File /opt/conda/lib/python3.9/site-packages/dx/utils/datatypes.py:205, in is_json_serializable(s)
    201 """
    202 Returns True if the object can be serialized to JSON.
    203 """
    204 try:
--> 205     s.to_json()
    206     return True
    207 except (TypeError, OverflowError, UnicodeDecodeError):

File /opt/conda/lib/python3.9/site-packages/pandas/core/generic.py:2621, in NDFrame.to_json(self, path_or_buf, orient, date_format, double_precision, force_ascii, date_unit, default_handler, lines, compression, index, indent, storage_options)
   2618 config.is_nonnegative_int(indent)
   2619 indent = indent or 0
-> 2621 return json.to_json(
   2622     path_or_buf=path_or_buf,
   2623     obj=self,
   2624     orient=orient,
   2625     date_format=date_format,
   2626     double_precision=double_precision,
   2627     force_ascii=force_ascii,
   2628     date_unit=date_unit,
   2629     default_handler=default_handler,
   2630     lines=lines,
   2631     compression=compression,
   2632     index=index,
   2633     indent=indent,
   2634     storage_options=storage_options,
   2635 )

File /opt/conda/lib/python3.9/site-packages/pandas/io/json/_json.py:110, in to_json(path_or_buf, obj, orient, date_format, double_precision, force_ascii, date_unit, default_handler, lines, compression, index, indent, storage_options)
    107 else:
    108     raise NotImplementedError("'obj' should be a Series or a DataFrame")
--> 110 s = writer(
    111     obj,
    112     orient=orient,
    113     date_format=date_format,
    114     double_precision=double_precision,
    115     ensure_ascii=force_ascii,
    116     date_unit=date_unit,
    117     default_handler=default_handler,
    118     index=index,
    119     indent=indent,
    120 ).write()
    122 if lines:
    123     s = convert_to_line_delimits(s)

File /opt/conda/lib/python3.9/site-packages/pandas/io/json/_json.py:165, in Writer.__init__(self, obj, orient, date_format, double_precision, ensure_ascii, date_unit, index, default_handler, indent)
    162 self.indent = indent
    164 self.is_copy = None
--> 165 self._format_axes()

File /opt/conda/lib/python3.9/site-packages/pandas/io/json/_json.py:202, in SeriesWriter._format_axes(self)
    200 def _format_axes(self):
    201     if not self.obj.index.is_unique and self.orient == "index":
--> 202         raise ValueError(f"Series index must be unique for orient='{self.orient}'")

ValueError: Series index must be unique for orient='index'