Because the category makes the protocol apply to all `NSMutableDictionaries`, it can't assume keys are `NSString`. Because these types are covariant, the change is not breaking.

TODO:
- Documentation
- Make public
- Check I’ve implemented all NSMutDict required methods

* Reference discussion: https://github.com/grpc/grpc/pull/4108#discussion-diff-44494393

For local running one can still tests AllTests, but for C.I. one can
test the 4 of them in parallel (and, importantly, deactivate the remote
interop tests until we fix them on Travis; while keeping the rest).

I had to make the remote tests a subclass of an abstract InteropTests
(like InteropTestsLocalSSL and InteropTestsLocalCleartext), and then
deactivate the abstract InteropTests on the 4 schemes that use it.

Eventually all samples will be tested.