Fix the (,) instance for PersistField to use the list instance

I found when i had models with `(Text, Text)` fields I was getting this error when storing a record and then trying to query for it:

```
14/Mar/2014:14:09:06 +1100 [Error#yesod-core] PersistMarshalError "field hash: Expected 2 item PersistList, received: PersistText \"[\\\"stext1\\\",\\\"stext2\\\"]\"" @(yesod-core-1.2.6.7:Yesod.Core.Class.Yesod ./Yesod/Core/Class/Yesod.hs:503:5)
```

It looks like this was a problem with the List instance for `PersistField` as well, and was fixed in [this](https://github.com/yesodweb/persistent/commit/32a9fb03df4f93fe9dd9bcff817019a56450a90a) commit but the tuple instance wasnt updated as well, I've changed the tuple instance to use the list instance.