JSONRenderer modifies data when status_code is `204 NO CONTENT`

[dig3]

JSONRenderer modifies the data to be renedered, from None to {'data': None} when response.status_code is 204.

From HTTP's RFC I see that:

All responses to the HEAD request method
MUST NOT include a message-body, even though the presence of entity-
header fields might lead one to believe they do. All 1xx
(informational), 204 (no content), and 304 (not modified) responses
MUST NOT include a message-body

This behavior made some clients to crash when the response stauts is 204 NO CONTENT and the header Content-Length is different from 0 (in this case 13 since the response body is {"data":null}).