proto: Unmarshal should silently skip unknown map fields

[awalterschulze]

https://github.com/golang/protobuf/blob/master/proto/decode.go#L868

I think we should, since maps should be interchangeable with repeated messages containing a key and value field. The messages with extra fields should also be compatible with maps, since they are compatible with messages with only a key and a value.

[dsnet]

I checked what the C++ implementation does:

• It ignores any tags other than 1 or 2.
• If tags 1 or 2 are missing, then it assumes the zero value.
• If a tag is repeated, the later takes precedence.
• It ignores tags with the wrong wire type (e.g., tag 1 with a wire type of Fixed32, when it expects Bytes).

This is entirely consistent with parsing each entry as a message.