Commit 5008423408 added support for
multipart/* media types in FormHttpMessageConverter, but users still had
to manually register multipart/mixed as a supported media type in order
to POST multipart data with that content type.
This commit removes the need to manually register multipart/mixed as a
supported media type by registering it automatically in
FormHttpMessageConverter. In addition, this commit introduces
MULTIPART_MIXED and MULTIPART_MIXED_VALUE constants in MediaType.
Closes gh-23209
896496341a