Need test cases for dynamic message's MergeFrom and MergeInto methods

[jhump]

Some of this logic is indirectly exercised via some of the tests (and the grpcurl project actually relies on these functions, and some of its tests exercise even more of the code).

But recently found bugs like addressed in #146 show that there are still some ugly corners in here. There are TODOs for test cases for these methods, and actually addressing these TODOs is rising in urgency.

[jhump]

Now that the new protobuf runtime includes a dynamic message implementation (in google.golang.org/protobuf/types/dynamicpb), I do not intend to add test coverage of my own package.

A future v2 of this repo will not include the desc and dynamic packages as that functionality is now provided in the core runtime.