Allow @RequestPart user-defined POJOs

[darrenfoong]

I'm using Spring Cloud OpenFeign, and I have a use case where I need to send multiple files and user-defined 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 #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:

• Updating SpringEncoder to (optionally) use a new SpringPojoFormEncoder that comes with a PojoFormWriter, which serializes user-defined POJOs.
• PojoFormWriter is abstract; developers can implement missing methods to serialize POJOs to any string format. In this PR, I implemented JsonPojoFormWriter, which uses Jackson.
• Update FeignClientsConfiguration to optionally autowire a PojoFormWriter that will be used with SpringEncoder upon application start-up.
• Update SpringMvcContract.processAnnotationsOnParameter to not expand parameters when the Content-Type is multipart/form-data. This is because a simple, single-field POJO like FeignClientTests.Hello would have been converted to a String.
• Add unit tests.

I am not sure how best to fix the issue in processAnnotationsOnParameter and I don't have a good understand of what it does exactly. I'd like to know a better way to prevent expansion of simple POJOs like FeignClientTests.Hello.

I had earlier created a pull request on feign-form (OpenFeign/feign-form#89), but considering that the project is not regularly maintained, I decided to make a PR here instead.

I had also noticed that the recommended usage on feign-form's documentation is to create a new SpringFormEncoder(new SpringEncoder(...)), but because SpringEncoder itself has an internal instance of SpringFormEncoder, I thought it would be cleaner to make a PR here.

[codecov]

Codecov Report

Merging #314 into 2.2.x will increase coverage by 0.17%.
The diff coverage is 83.67%.

@@             Coverage Diff              @@
##              2.2.x     #314      +/-   ##
============================================
+ Coverage     78.84%   79.01%   +0.17%     
- Complexity      396      411      +15     
============================================
  Files            52       54       +2     
  Lines          1541     1587      +46     
  Branches        223      230       +7     
============================================
+ Hits           1215     1254      +39     
- Misses          233      237       +4     
- Partials         93       96       +3     

Impacted Files	Coverage Δ	Complexity Δ
...rk/cloud/openfeign/support/AbstractFormWriter.java	72.72% <72.72%> (ø)	6.00 <6.00> (?)
...ork/cloud/openfeign/support/SpringMvcContract.java	88.82% <87.50%> (-0.13%)	56.00 <2.00> (+3.00)	⬇️
...ork/cloud/openfeign/FeignClientsConfiguration.java	97.22% <90.90%> (+1.06%)	13.00 <2.00> (+2.00)
...mework/cloud/openfeign/support/JsonFormWriter.java	100.00% <100.00%> (ø)	3.00 <3.00> (?)
...amework/cloud/openfeign/support/SpringEncoder.java	90.27% <100.00%> (+0.57%)	18.00 <1.00> (+1.00)

[spencergibb]

I'll let @OlgaMaciaszek comment on actual functionality as opposed to just structure and naming.

[OlgaMaciaszek]

@darrenfoong Thanks for submitting this. The logic looks good to me, but I've added some comments for you to address.

[OlgaMaciaszek]

LGTM

[darrenfoong]

Thanks @spencergibb and @OlgaMaciaszek for reviewing and merging this PR!