Beef up the internal server error class

* Add tests for it
* Ensure we set a Content-Type header (Fixes #42)

Signed-off-by: Alistair Scott <alistair@rightscale.com>
oops, just realized that this whole definition block was cut out of the http.rb file? ... that should still be there as it registers all the defined response classes as response templates. So can you paste it back please?