remove any expectations for received xml:lang tag

the way i understand rfc 6120, clients will not have any assumptions
about the received xml:lang tag

source/Network/Xmpp/Stream.hs

@@ -120,11 +120,10 @@ xmppStream expectedTo = do
             Right r -> validateData r
 
     validateData (_, _, _, _, Nothing) = throwError $ StreamWrongLangTag Nothing
-    validateData (ver, from, to, i, Just lang)
+    validateData (ver, from, to, i, lang)
       | ver /= "1.0" = throwError $ StreamWrongVersion (Just ver)
       | isJust to && to /= expectedTo = throwError $ StreamWrongTo (Text.pack . show <$> to)
---      | lang /= expectedLang = throwError $ StreamWrongLangTag lang
-      | otherwise        = return (from, to, i, lang)
+      | otherwise = return (from, to, i, fromJust lang)
     xmppStreamFeatures :: StreamSink ServerFeatures
     xmppStreamFeatures = do
         e <- lift $ elements =$ CL.head