BUG: fixes 'string_types' encoding failure with unicode cols in py2

As part of warning check, object type of potential attributes was checked
for subtypes of pd.compat.str_types before being checked for overlap with
methods defined on ndframes. This causes decode errors in Python2 when
users attempt to add columns with unicode column names. Fix is to compare
against `str`.

Author: deniederhut

Diff for: pandas/core/generic.py

@@ -1906,8 +1906,7 @@ def _slice(self, slobj, axis=0, kind=None):
         return result
 
     def _set_item(self, key, value):
-        if (isinstance(key, string_types)
-            and callable(getattr(self, key, None))):
+        if isinstance(key, str) and callable(getattr(self, key, None)):
             warnings.warn("Column name '{key}' collides with a built-in "
                           "method, which will cause unexpected attribute "
                           "behavior".format(key=key), stacklevel=3)