Pass the GET request of /staging/(buildpack_cache|droplets)/ to cloud_controller directly

The problem described here:
https://groups.google.com/a/cloudfoundry.org/forum/?fromgroups=#!topic/vcap-dev/PZLovYnQrS0

DEA can not download the staged application as it gets a 405 error from nginx and the GET request never goes to cloud_controller.

Download(GET) should not goes to the upload module. This pull request can solve it.