ut bytes in the response to set the Content-Length header accurately.

 <p>
 If you override this method, you can avoid computing the response body and just set the response headers directly to
 improve performance. Make sure that the <code>doHead</code> method you write is both safe and idempotent (that is,
 protects itself from being called multiple times for one HTTP HEAD request).

 <p>
 If the HTTP HEAD request is incorrectly formatted, <code>doHead</code> returns an HTTP "Bad Request" message.</div>
<