Replace call to fetchAccessToken with fetchAccessToken2

This comment comes from hoauth2:

-- OAuth2 spec allows `client_id` and `client_secret` to
-- either be sent in the header (as basic authentication)
-- OR as form/url params.
-- The OAuth server can choose to implement only one, or both.
-- Unfortunately, there is no way for the OAuth client (i.e. this library) to
-- know which method to use. Please take a look at the documentation of the
-- service that you are integrating with and either use `fetchAccessToken` or `fetchAccessToken2`

`fetchAccessToken2` is a drop-in replacement for `fetchAccessToken` that just adds `client_id` and `client_secret` to the body as form parameters, as permitted by [RFC 6749](https://tools.ietf.org/html/rfc6749#section-2.3.1). Some authorization server implementations only accept client credentials in this form.

src/Yesod/Auth/OAuth2/Dispatch.hs

@@ -67,7 +67,7 @@ dispatchCallback name oauth2 getCreds = do
     code <- requireGetParam "code"
     manager <- authHttpManager
     oauth2' <- withCallbackAndState name oauth2 csrf
-    token <- errLeft $ fetchAccessToken manager oauth2' $ ExchangeToken code
+    token <- errLeft $ fetchAccessToken2 manager oauth2' $ ExchangeToken code
     creds <- errLeft $ tryFetchCreds $ getCreds manager token
     setCredsRedirect creds
   where