Cannot proxy Docker repos on Bintray

When Nexus makes upstream Docker requests (e.g. https://your-org-your-repo.bintray.io/v2/your-package/manifests/latest), Bintray responds with a `406/Not Acceptable`.

After much experimentation, we determined this can be resolved in any of the following ways:

• Send no `Accept` headers.  Currently, Nexus sends:
  Accept:application/vnd.docker.distribution.manifest.v2+json
  Accept:application/vnd.docker.distribution.manifest.v1+prettyjws
  Accept:application/vnd.docker.distribution.manifest.v1+json
• In addition to these, send:
  Accept:application/json
• Send preemptive auth.

It looks as though the Docker CLI doesn't send any `Accept` headers, so this could be the easiest option.

Expected

The docker CLI does work against bintray. NXRM should act like docker CLI does as close as possible.