adding requestHeaderHost to Request. (#190)

wai/Network/Wai.hs

@@ -186,4 +186,5 @@ defaultRequest = Request
     , requestBody = return ()
     , vault = mempty
     , requestBodyLength = KnownLength 0
+    , requestHeaderHost = Nothing
     }

wai/Network/Wai/Internal.hs

@@ -58,6 +58,8 @@ data Request = Request
   --
   -- Since 1.4.0
   , requestBodyLength     :: RequestBodyLength
+  -- | The value of the Host header in a HTTP request.
+  , requestHeaderHost     :: Maybe B.ByteString
   }
   deriving (Typeable)