waiProxy: Remove Content-Length for Encoded Responses

This is a proper fix to #45.

In previous merge request (#45), for HTTP/2 connections, Content-Length
header is retained in order to avoid problems with Docker client rejecting
chunked encoding responses from Docker registry.

However, this approach failed when the upstream response was encoded
(e.g. with 'Content-Encoding: gzip' in header). The retained
'Content-Length' header then mismatched the actual content size after
decoding.

To fix this, we now strip the Content-Length header if Content-Encoding
header is present, along with our existing checks for cases where chunked
encoding is not used (HTTP/2 and HEAD requests).

Network/HTTP/ReverseProxy.hs

@@ -54,7 +54,7 @@ import           Data.Functor.Identity          (Identity (..))
 import           Data.IORef
 import           Data.List.NonEmpty             (NonEmpty (..))
 import qualified Data.List.NonEmpty             as NE
-import           Data.Maybe                     (fromMaybe, listToMaybe)
+import           Data.Maybe                     (fromMaybe, isNothing, listToMaybe)
 import           Data.Monoid                    (mappend, mconcat, (<>))
 import           Data.Set                       (Set)
 import qualified Data.Set                       as Set
@@ -426,12 +426,14 @@ waiProxyToSettings getDest wps' manager req0 sendResponse = do
                                         (awaitForever (\bs -> yield (Chunk $ fromByteString bs) >> yield Flush))
                                         (wpsProcessBody wps req $ const () <$> res)
                             src = bodyReaderSource $ HC.responseBody res
-                            noChunked = HT.httpMajor (WAI.httpVersion req) >= 2 || WAI.requestMethod req == HT.methodHead
+                            headers = HC.responseHeaders res
+                            notEncoded = isNothing (lookup "content-encoding" headers)
+                            notChunked = HT.httpMajor (WAI.httpVersion req) >= 2 || WAI.requestMethod req == HT.methodHead
                         sendResponse $ WAI.responseStream
                             (HC.responseStatus res)
                             (filter (\(key, v) -> not (key `Set.member` strippedHeaders) ||
-                                                  key == "content-length" && (noChunked || v == "0"))
-                                    (HC.responseHeaders res))
+                                                  key == "content-length" && (notEncoded && notChunked || v == "0"))
+                                    headers)
                             (\sendChunk flush -> runConduit $ src .| conduit .| CL.mapM_ (\mb ->
                                 case mb of
                                     Flush -> flush