tests: improve test coverage responseconverterutils

This PR brings unit test improvements for ResponseConverterUtils and ResponseUtils classes.

Notes:

I took the liberty refactoring the tests into AAA format (without changing what and how they do), and moving the input parameters into the tests, renaming the methods and removing documentation headers. As the volume of test cases grew it was difficult to scroll back and fort between the method and the input values placed on the top of the file.
I also renamed the methods of the test service so they tells more about which underlying ResponseConverterUtils method is called
I extended the already implemented positive test cases with negative tests for having some coverage on corner cases
there are seemingly superflous test cases, I added these to document the existing behavior
streaming is not covered yet, it will be done by a later PR

Coverage before PR:

Coverage by this PR:

IBM / java-sdk-core