Alter how runtime.SerializeInfo is represented to simplify negotiation
and reduce the need to allocate during negotiation. Simplify the dynamic
client's logic around negotiating type. Add more tests for media type
handling where necessary.
This is so that we can use NewAPIFactory() from cmd/set/*test.go
Up until now we would get a import loop error.
This commit also adds a basic unit test case for cmd/set/set_image.go
