Spring Fix ClassCastException from return value validation with proxy when adaptConstraintViolations=true

Fixed method arguments of applyReturnValueValidation to be called with the return value, no the method arguments. Added some tests with "adaptConstraintViolations=true" which also failed before the fix.

But I'm not into reactive programming, so I couln't figure out a way to do return value validation on Monos.

Comment From: sbluemel

@rstoyanchev I had some time to look into it as well. But I'm unsure about the reactive tests. Please have a look.

Comment From: sbluemel

There are now similiar tests in the original pull request by @imvtsl.