Don't try to scan content on HEAD requests.
Don't try to scan content on HEAD requests.

Scanning an empty response body doesn't sound like it would do any harm,
but it is useless, and it was causing the Content-Length header to be
set to 0 instead of passing the original Content-Length through.

Change-Id: I7bba97492d179532f2235d3814f8c9fa223dbda2

andybalholm/redwood,unixabg/redwood,unixabg/redwood,andybalholm/redwood