Fix compile error in v0.9

While trying to install yesod-comments, I got this error:
```
Yesod/Comments/Storage.hs:32:1:
    Couldn't match expected type `[Char]' with actual type `Text'
    In the return type of a call of persistent-template-1.2.0.2:Database.Persist.TH.pack'
    In the second argument of `Data.Monoid.mappend', namely
      `(persistent-template-1.2.0.2:Database.Persist.TH.pack'
          "threadId")'
    In the first argument of `Data.Monoid.mappend', namely
      `("field "
        `Data.Monoid.mappend`
          (persistent-template-1.2.0.2:Database.Persist.TH.pack'
             "threadId"))'
cabal: Error: some packages failed to install:
yesod-comments-0.9.0 failed during the building phase. The exception was:
ExitFailure 1
```
To replicate, ``cabal update && cabal install yesod-comments``

This fixes it for me, but, I'm just a newbie at Haskell so what do I know!