message: check that target_column is six.string_types
diff: @@ -96,8 +96,8 @@ def pandas_input_fn(x,
  raise TypeError('shuffle must be explicitly set as boolean; '
  'got {}'.format(shuffle))
  - if isinstance(target_column, list):
  - raise TypeError('target_column must be a string or None')
  + if not isinstance(target_column, six.string_types):
  + raise TypeError('target_column must be a string type')
  x = x.copy()
  if y is not None:
  