Responses to HEAD requests _should_ have a Content-Length header
that's identical to GET requests. When a body is provided by the
application, set the Content-Length header to the size of the body
and replace the body with an empty Array; when no body is provided
by the application in response to a HEAD request, and the
Content-Length is calculated to be "0", remove the Content-Length
header entirely to avoid mis-matched values.
The idea here is that it's better to omit the header when we believe
the Content-Length is not indicative of the same in response to GET
than to send the Content-Length with a mismatched value. Logic
taken from Apache and is generally in line with RFC 2616.
Ryan Tomayko