HTTP request snippet for PATCH, POST and PUT requests with no body has an incorrectly inferred Content-Type header

[banseok1216]

Closes #1017

Summary

REST Docs previously inferred Content-Type: application/x-www-form-urlencoded
when processing POST/PUT/PATCH requests that had no Content-Type, no body, and no parts.

This caused the generated documentation to differ from the real request seen by Spring MVC.

What this PR changes

• Removes the fallback inference of application/x-www-form-urlencoded
• Content-Type is now documented only when:
  • explicitly provided, or
  • required for multipart requests
• Ensures requests with no Content-Type, no content, and no parts
  produce no Content-Type header in the generated snippet

Rationale

Documentation should reflect the exact request sent by the client.
HTTP provides no default media type, and Spring MVC does not infer one.
Avoiding implicit Content-Type injection ensures accuracy and prevents misleading API documentation.

[wilkinsona]

Thanks for the PR. Setting the Content-Type header for a PATCH, POST, or PUT request that has no body certainly seems wrong and I think that should be corrected. I am not so sure about removing it entirely as I wonder if it could be a breaking change for some situations. For 3.0.x at least, I think it would be better to only remove the setting of the Content-Type header for requests with no body content. A separate change that removes the inference entirely could then be considered for 4.0.x.

[banseok1216]

Thanks for the feedback. The PR has been updated as suggested.
We can look at removing the inference entirely in 4.0.x when the time comes.

[wilkinsona]

This test passes both with and without the proposed change to HttpRequestSnippet so I don't think I understand its intent. Given that the bug occurs when there's no content type header in the request, why is this new test creating a request with a content type header?

[banseok1216]

Sorry, that was my mistake — while verifying the fix I ended up adding the wrong test.
I replace the tests to cover the actual bug case.