Compression is flaky when using Metadata in call

[mwitkow]

You can easily make the TestCompressOK test fail by adding client-side metadata.

    ctx := metadata.NewContext(context.Background(), metadata.Pairs("something", "something"))
    if _, err := tc.UnaryCall(ctx, req); err != nil {
        t.Fatalf("TestService/UnaryCall(_, _) = _, %v, want _, <nil>", err)
    }

This has been a massive time sink in #683. It seems that the sendCompress secition in code in http2Server.WriteHeader never gets called

    if s.sendCompress != "" {
        fmt.Printf("Calling send compress!\n")
        t.hEnc.WriteField(hpack.HeaderField{Name: "grpc-encoding", Value: s.sendCompress})
    }

[mwitkow]

The problem is that the grpc-encoding client doesn't receive a grpc-encoding header.

[mwitkow]

The problem is:
In Server.processUnaryRPC the call to set the compression mode of the stream ( stream.SetSendCompress(s.opts.cp.Type())) is made too late, and some SendHeader calls already completed.