Add support for Enum<?> as Feign client parameters for multipart payload

[reta]

Hey guys,

We've run into an issue recently when multipart endpoint uses Java's enumeration (Enum<?>) for some of the parameter. The MultipartFormContentProcessor does recognize the Enum as the POJO and tries to convert it into the Map, ending up with the empty one. As such, all Enum<?> parameters are not being included into the request.

The fix is simple (add EnumWriter), the test cases is provided, easy to reproduce.
Thank you.

Best Regards,
Andriy Redko

[reta]

Sorry @xxlabaza, may you take a look please? Thank a lot :)