Stop moving session language ahead of reqLangs

Yesod.Core.Handler.languages checks first for a language set in the
user's session, prepending that value to YesodRequest{reqLangs}, so it
is respected above all else if present.

For context, reqLangs itself also includes the session, but just later
in line:

    langs' = catMaybes [ lookup langKey gets -- Query _LANG
                       , lookup langKey cookies     -- Cookie _LANG
                       , lookupText langKey session -- Session _LANG
                       ] ++ langs                    -- Accept-Language(s)

In #1720, it was raised that allowing the session (something implicitly
present for any request) to override a query parameter (something
explicitly given on that request) is surprising.

We decided (without knowing what order reqLangs was doing) that query,
cookie, session, accept was best and languages should be changed to do
that. Conveniently, this just makes languages equivalent to reqLangs, so
that is what this patch does.

yesod-core/src/Yesod/Core/Handler.hs

@@ -1226,10 +1226,10 @@ cacheBySet key value = do
 -- Languages are determined based on the following (in descending order
 -- of preference):
 --
--- * The _LANG user session variable.
---
 -- * The _LANG get parameter.
 --
+-- * The _LANG user session variable.
+--
 -- * The _LANG cookie.
 --
 -- * Accept-Language HTTP header.
@@ -1239,10 +1239,7 @@ cacheBySet key value = do
 --
 -- This is handled by parseWaiRequest (not exposed).
 languages :: MonadHandler m => m [Text]
-languages = do
-    mlang <- lookupSession langKey
-    langs <- reqLangs <$> getRequest
-    return $ maybe id (:) mlang langs
+languages = reqLangs <$> getRequest
 
 lookup' :: Eq a => a -> [(a, b)] -> [b]
 lookup' a = map snd . filter (\x -> a == fst x)