grpc-web: trailers message is not being decompressed when compressed bit is set

[jhump]

The final message in a grpc-web response stream has a special bit set that indicates it contains trailers. If the header byte for that message has both the "compressed" and "trailers" bits set, the compressed flag is effectively ignored. It attempts to parse the compressed data, which of course results in an error:

InvalidOperationException: Error parsing badly formatted trailing header.

Having both bits set is definitely valid per the protocol. The protocol spec even has exactly this combination in the examples for header byte.

[jhump]

@sudorandom created a repro case for this bug here: https://github.com/sudorandom/dotnet-grpc-compress-bug