Need hooks for testing

[dgrijalva]

While trying to unit test my own request filters, I realized it's very difficult as I can't create my own complete falcore.Request objects.

[smw1218]

This was a bit tricky since the request needed to do some bookkeeping around the filter call, added a new func TestWithRequest(request _http.Request, filter RequestFilter) (_Request, *http.Response).

I hope this is enough to enable testing since the lifecycle stuff would be messy to expose and exposing the internal newRequest would return a mostly broken object.