Test json binding

Implement a simple test to verify that binding of JSON requests are handled by the Json function. To accomplish this, I had to rename the existing Json method to DefaultJsonBinder and introduce a function variable, Json, whose default value is DefaultJsonBinder.

I'm happy to do this another way, or for the PR to not be merged at all, but since I did this work while getting started on the #9, I figured I'd submit it. Since it's ultimately completely orthogonal to #9, I didn't want to mix the two concerns.