update syntax of form_for to rails 3.2.8. Fix "message cannot be replied by Admin"

There are two more locations where form_for seem like a bug, they are in 'app/views/site/login.html.haml' However, I does not have a mean to test it. (Dunno how to use site)

(grafted from 17d48b004affe1e1dd4a981d9beef9d61ce83078)

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extra : source : 17d48b004affe1e1dd4a981d9beef9d61ce83078