Add the "grpc-encoding" header to the response (#20)

* Add the "grpc-encoding" header to the response

Once the per-message compression encoding has been negotiated (by
inspecting the `grpc-accept-encoding` request header), the server should
tell the client the chosen encoding (either `identity` or `gzip`) by
setting the `grpc-encoding` response header.

The meaning of the header is: if any message in the response has its
compression flag set, then that message has been compressed using this
encoding.

Some clients can live with the header being missing. If they encounter a
message with its compression flag set, they just assume the encoding is
GZip. But other clients, notably the official Java gRPC library, are
stricter and require the header to be present.

Aside: compression can be done at the stream level rather than the
individual message level. In that case, the `content-encoding` header
must be set, and the `grpc-encoding` header must *not* be set.

None of this is really documented anywhere. I worked it out using
Wireshark and reading the [grpc-java
source](https://github.com/grpc/grpc-java/blob/8d4240f4800f20de0e5f0b7c213577b3c0324f93/core/src/main/java/io/grpc/internal/AbstractClientStream.java#L313-L356).

warp-grpc/src/Network/GRPC/Server/Wai.hs

@@ -95,28 +95,27 @@ grpcService compressions services app = \req rep -> do
                               , Handler $ \(e::SomeException) -> 
                                  modifyGRPCStatus r req (GRPCStatus INTERNAL $ ByteString.pack $ show e)
                               ]
-            in (rep $ responseStream status200 hdrs200 grpcHandler)
+            in (rep $ responseStream status200 (hdrs200 req) grpcHandler)
         Nothing ->
             app req rep
   where
-    hdrs200 = [
+    bestCompression req   = lookupEncoding req compressions
+    pickedCompression req = fromMaybe (Encoding uncompressed) (bestCompression req)
+
+    hopefulDecompression req = lookupDecoding req compressions
+    pickedDecompression req  = fromMaybe (Decoding uncompressed) (hopefulDecompression req)
+
+    hdrs200 req = [
         ("content-type", grpcContentTypeHV)
+      , ("grpc-encoding", grpcCompressionHV (_getEncodingCompression $ pickedCompression req))
       , ("trailer", CI.original grpcStatusH)
       , ("trailer", CI.original grpcMessageH)
       ]
     lookupHandler :: ByteString -> [ServiceHandler] -> Maybe WaiHandler
     lookupHandler p plainHandlers = grpcWaiHandler <$>
         List.find (\(ServiceHandler rpcPath _) -> rpcPath == p) plainHandlers
     doHandle r handler req write flush = do
-        let bestCompression = lookupEncoding req compressions
-        let pickedCompression = fromMaybe (Encoding uncompressed) bestCompression
-
-        let hopefulDecompression = lookupDecoding req compressions
-        let pickedDecompression = fromMaybe (Decoding uncompressed) hopefulDecompression
-
-        putStrLn "running handler"
-        _ <- handler pickedDecompression pickedCompression req write flush
-        putStrLn "setting GRPC status"
+        _ <- handler (pickedDecompression req) (pickedCompression req) req write flush
         modifyGRPCStatus r req (GRPCStatus OK "WAI handler ended.")
 
 #if MIN_VERSION_warp(3,3,0)