BooleanField default values not used when an empty POST is sent

[callorico]

Seems related to #3811 and #3788.

When I make a POST with an empty body to a view that is using a serializer that contains a BooleanField that has its default kwarg set to a callable, I am getting the following validation error returned:

{"flag":["\"<function <lambda> at 0x1069fbb18>\" is not a valid boolean."]}

Even though I am explicitly setting the content type in the request to application/json, the html.is_html_input method is returning true on the data dict. This causes the BooleanField.default_empty_html value to be returned from BooleanField.get_value. default_empty_html seems to be set to the Field's default value in this case which is the callable object. This produces a rather strange validation error message.

Failing test case here: #3891