gRPC: special handling of the TE request header. According to the gRPC protocol specification, the "TE" header is used to detect incompatible proxies, and at least grpc-c server rejects requests without "TE: trailers". To preserve the logic, we have to pass "TE: trailers" to the backend if and only if the original request contains "trailers" in the "TE" header. Note that no other TE values are allowed in HTTP/2, so we have to remove anything else.