Django REST Framework ignores the Content-Type on request body parts, so
it doesn't know that they are JSON-encoded. Instead, it just tries to
decode each part as if it was an `str()`-encoded value.
Change the encoding to match the decoding. The only type this matters
for is `str`, because `json.dumps` and `str` produce different encodings
for `str` values.
Remove `none_type` from the list of encodable types since, to my
knowledge, there's no way to encode a `None` value as a
`multipart/form-data` part in a way that DRF will understand.
Roman Donchenko
Maxim Zhiltsov