Issues found when implementing grpc/grpc-experiments#243.

• Lets set an immutable dictionary.
• Lets just set single headers without checking first if the dictionary is nil.
• Surfaces trailers.
• Puts response metadata into NSError.
• Drops duplicate response metadata so users don't have to write call.responseMetadata[@"my-header"][0] in the common case.
• Adds metadata unit tests.
• Fixes specification in GRPCCall.h.

I think in the process we might have gotten rid of a race condition for setting the response headers.