accept missing to header from server

Interoperability Note: It is possible that implementations based on
[RFC3920] will not include the 'to' address on stream headers; an entity
SHOULD be liberal in accepting such stream headers.

source/Network/Xmpp/Stream.hs

@@ -122,7 +122,7 @@ xmppStream expectedTo = do
     validateData (_, _, _, _, Nothing) = throwError $ StreamWrongLangTag Nothing
     validateData (ver, from, to, i, Just lang)
       | ver /= "1.0" = throwError $ StreamWrongVersion (Just ver)
-      | to /= expectedTo = throwError $ StreamWrongTo (Text.pack . show <$> to)
+      | isJust to && to /= expectedTo = throwError $ StreamWrongTo (Text.pack . show <$> to)
 --      | lang /= expectedLang = throwError $ StreamWrongLangTag lang
       | otherwise        = return (from, to, i, lang)
     xmppStreamFeatures :: StreamSink ServerFeatures