w605 fixes for identical tokens

An infinite loop was occuring when two w605 tokens appeared on the same line because the string find() function would only return the position of the first instance to be fixed.  The Token datastrucute (tuple) already contained the start position where the fix was needed, so it is used directly instead of doing a search.  Unit tests added that catch the original issue.

autopep8.py

@@ -1363,7 +1363,7 @@ def get_w605_position(tokens):
         'N', 'u', 'U',
     ]
 
-    for token_type, text, start, end, line in tokens:
+    for token_type, text, start_pos, end_pos, line in tokens:
         if token_type == tokenize.STRING:
             quote = text[-3:] if text[-3:] in ('"""', "'''") else text[-1]
             # Extract string modifiers (e.g. u or r)
@@ -1378,7 +1378,8 @@ def get_w605_position(tokens):
                     pos += 1
                     if string[pos] not in valid:
                         yield (
-                            line.find(text),
+                            # No need to search line, token stores position
+                            start_pos[1],
                             "W605 invalid escape sequence '\\%s'" %
                             string[pos],
                         )

test/test_autopep8.py

@@ -4734,6 +4734,25 @@ def test_w605_simple(self):
         with autopep8_context(line, options=['--aggressive']) as result:
             self.assertEqual(fixed, result)
 
+    def test_w605_identical_token(self):
+        # ***NOTE***: The --pep8-passes option is requred to prevent an infinite loop in
+        # the old, failing code. DO NOT REMOVE.
+        line = "escape = foo('\.bar', '\.kilroy')\n"
+        fixed = "escape = foo(r'\.bar', r'\.kilroy')\n"
+        with autopep8_context(line, options=['--aggressive', '--pep8-passes', '5']) as result:
+            self.assertEqual(fixed, result, "Two tokens get r added")
+
+        line = "escape = foo('\.bar', r'\.kilroy')\n"
+        fixed = "escape = foo(r'\.bar', r'\.kilroy')\n"
+        with autopep8_context(line, options=['--aggressive', '--pep8-passes', '5']) as result:
+            self.assertEqual(fixed, result, "r not added if already there")
+
+        # Test Case to catch bad behavior reported in Issue #449
+        line = "escape = foo('\.bar', '\.bar')\n"
+        fixed = "escape = foo(r'\.bar', r'\.bar')\n"
+        with autopep8_context(line, options=['--aggressive', '--pep8-passes', '5']) as result:
+            self.assertEqual(fixed, result)
+
     def test_trailing_whitespace_in_multiline_string(self):
         line = 'x = """ \nhello"""    \n'
         fixed = 'x = """ \nhello"""\n'