Allow @RequestPart POJOs

[darrenfoong]

I'm using Spring Cloud OpenFeign, and I have a use case where I need to send multiple files and POJOs as multipart/form-data, for example:

@PostMapping("/items")
SomeResponse uploadItems(@RequestPart List<SomePojo> items, @RequestPart List<MultipartFile> itemImages);

There was a recent fix in spring-cloud/spring-cloud-openfeign#258 that allowed the use of multiple @RequestParts, but it was only for MultipartFile and boxed primitive types. This PR builds on the abovementioned PR by:

• Upgrading the Spring dependencies in pom.xml to enjoy the benefits of the abovementioned PR
• Add a PojoSerializationWriter that can serialize POJOs to some string; in the unit tests I use Jackson to serialize as JSON
• Update SpringFormEncoder to (optionally) use PojoSerializationWriter
• Simplify SpringFormEncoder.encode() by removing code that is unnecessary due to Fix GH-201: Multiple @RequestPart not Working spring-cloud/spring-cloud-openfeign#258 already doing the conversion to Map (except for one if-clause for @ResponseBody MultipartFile)
• Update existing unit tests because Fix GH-201: Multiple @RequestPart not Working spring-cloud/spring-cloud-openfeign#258 requires @RequestParts to have a value
• Add new unit tests to test upload of various types of @RequestParts: collections, arrays, MultipartFile, etc.

With this PR, pull requests #71 and #74 are also no longer necessary.

[darrenfoong]

Moved the PR to spring-cloud/spring-cloud-openfeign#314