Clear the content-length header when not sending the body

The body is not sent for 502 errors, but unicorn was waiting for
data since the content-length header was set. Deadlock ensures,
and then someone (unicorn master?) terminates the request after
30 seconds. DLTP-184.
Merge pull request #125 from ndlib/nginx-502-bugfix

Clear the content-length header when not sending the body