Should treat absence of grpc-encoding header as identity

[FinleyMcIlwaine]

While testing against the go implementation with --deflate on the demo-client, the client succeeded even though the server did not compress the response because the go server did not set grpc-encoding: identity.

Compression negotiation should happen as if the header were present.

[edsko]

Closing this as WONTFIX, because we changed policy here: identity encoding is now always allowed. We previously did not do this, with as rationale that if the client provided a specific lists of encodings that did not include identity we should respect that. However, we anyway we have the option of deciding per message whether to encode it or not, independent of the negotiated encoding algorithm.