HttpMessageWriter for writing a MultiValueMap<String, ?>
as multipart form data, i.e. "multipart/form-data", to the body
of a request.
The serialization of individual parts is delegated to other writers.
By default only String and Resource parts are supported but
you can configure others through a constructor argument.
This writer can be configured with a FormHttpMessageWriter to
delegate to. It is the preferred way of supporting both form data and
multipart data (as opposed to registering each writer separately) so that
when the MediaType is not specified and generics are not present on
the target element type, we can inspect the values in the actual map and
decide whether to write plain form data (String values only) or otherwise.