Add test cases for invalid callback with JSON and non-JSON content-types.
Add test cases for invalid callback with JSON and non-JSON content-types.

tobiashm/rack-contrib,gferguson-gd/rack-contrib,vivrass/rack-contrib,gferguson-gd/rack-contrib,vivrass/rack-contrib,edmorley/rack-contrib,12spokes/rack-contrib,tobiashm/rack-contrib,edmorley/rack-contrib