Created unit tests for 

CollectionUtils.toMultiValueMap

as suggested by @jhoeller in gh-25140

I just followed the instructions given there although I could imagine a better solution which is a lot more DRY.

I will create a second PR for comparison.

I would have 2 small questions:

  1. The code actually being tested lies in
CollectionUtils

but the test I have named CollectionUtilsMultiValueMapAdapterTests.

Is this OK with you or are you following a strong name convention, thus I should move the tests to CollectionUtilsTest ?

  1. Is the gh issue number ok or should I create a separate one?

Comment From: midumitrescu

in favor of gh-25160