BUG: fillna returns frame when inplace=True if value is a dict (pandas-dev#16156)

Author: ante012

Diff for: doc/source/whatsnew/v0.21.0.txt

@@ -391,3 +391,4 @@ Other
 - Bug in :func:`eval` where the ``inplace`` parameter was being incorrectly handled (:issue:`16732`)
 - Bug in ``.isin()`` in which checking membership in empty ``Series`` objects raised an error (:issue:`16991`)
 - Bug in :func:`unique` where checking a tuple of strings raised a ``TypeError`` (:issue:`17108`)
+- Bug in :func: `fillna` returns frame when ``inplace=True`` and ``value`` is dict (:issue:`16156`)