HEAD on account returns 410 if account was deleted and not yet reaped

 * updated CONTRIBUTING.md
 * improved unit tests
 * 100% coverage for proxy acct controller tests
 * invoke fake_http_connect correctly

Change-Id: I0826c5a1c52efdd5ae95f7fde8024f2bff0751ba
Merge "HEAD on account returns 410 if account was deleted and not yet reaped"