update() produces 'invalid type promotion' error when used with datetime columns and overwrite=False

[Sereger13]

xref #3025

To reproduce:

    import pandas as pd
    df = pd.DataFrame({'A': [1, 2, 3],
                       'B': ['20140101', nan, '20140101'],
                       'C': [nan, nan, nan]})
    df2 = pd.DataFrame({'A': [1, 2],
                       'B': ['20140104', '20140105'],
                       })
    df.B = pd.to_datetime(df.B, coerce=True)
    df2.B = pd.to_datetime(df2.B, coerce=True)
    df.set_index('A', inplace=True)
    df2.set_index('A', inplace=True)

    #df.update(df2[['B']]) # Works fine
    df.update(df2[['B']], overwrite=False) # Fails with 'invalid type promotion' error.

[jreback]

can u pd.show_versions()

[Sereger13]

INSTALLED VERSIONS

commit: None
python: 2.7.5.final.0
python-bits: 32
OS: Linux
OS-release: 2.6.18-238.9.1.el5
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US

pandas: 0.15.0
nose: 1.3.0
Cython: 0.20
numpy: 1.7.1
scipy: 0.13.0
statsmodels: 0.5.0
IPython: 2.2.0
sphinx: 1.1.3
patsy: 0.2.1
dateutil: 1.5
pytz: 2013b
bottleneck: None
tables: 3.1.0
numexpr: 2.2.2
matplotlib: 1.3.1
openpyxl: 1.6.2
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: 0.5.5
lxml: None
bs4: 4.3.1
html5lib: None
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: 0.8.3
pymysql: None
psycopg2: None

[jreback]

this is 'caused' by #3025, IOW. the current machinery is doesn't handle these cases (and why #3025 is neeeded). so closing this and will xref to that issue.