[Java][Spring] Formdata parameter fix

[demtnt]

As per documentation https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/web/bind/annotation/RequestPart.html @RequestParam should be used instead of @requestpart if not file.

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Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/swagger-api/swagger-codegen/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/swagger-api/swagger-codegen/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[wing328]

cc @bbdouglas @JFCote @sreeshas @jfiala @lukoyanov @cbornet

[cbornet]

@demtnt please update the samples for spring

[wing328]

Annotation that can be used to associate the part of a "multipart/form-data" request with a method argument.

@demtnt What if the request content-type is x-www-form-urlencoded? Would this change still be applicable?

[wing328]

Related: #6438

[wing328]

Tests passed via https://circleci.com/gh/swagger-api/swagger-codegen/2778