Fix TestHelper not working when no parameters are given

I checked out the master with the recent @keithpitt fix to the TestHelper. Everything worked fine expect for requests without parameters, typically, my "get :index" request.

This comes from the fact that when no parameters are given, we set the parameters variable as an empty hash so It loose it's reference to the http_method variable.

Explicitly passing arguments prevent this issue while avoiding tweaking too much the process method.

Tested against rails 3.2.12 and 4.0.0.beta1