Spring: support all media-types for consumes

[iazel]

Problem

In Spring, we require the usage of @RequestBody annotation before we actually check for consumes in @RequestMapping (or any variation of that). This is quite limiting because @RequestBody only supports application/json and application/xml media-type.
For example, it is currently impossible to use mutlipart/form-data media type.

Solution

Only check for @RequestBody in case there is no consumes annotation already provided in RequestMapping.

Potential improvements

We could make it a little more smart and set it to multipart/form-data in case we use @RequestPart or one of the parameter if of type MultipartFile. I honestly think this is more of an edge case and probably it's not worth it to add this extra complexity, but let me know what you think :)

[K0nstantine]

We also need this feature. Also the "@RequestPart" improvement.

[cc-jhr]

Hi @iazel,

thank you for providing this PR.
Looks good to me.