Request to add a ToTypedContent to the response tyvar of Handler for yesodMiddleware.

The yesodMiddleware would be a lot more useful if there was a way to interact with the response. I looked at where the middleware was run, and the Handler has a ToTypeContent constraint on it, so I don't see the harm of adding one, and the gain is very large.

Without this contraint or something similar I am forced to use Wai middleware. Not ideal.