added modify_response_headers hook

doc/hacking/hooks.txt

@@ -92,6 +92,12 @@ Called when we're about to serve a local file, before we've done any
 work.  You can change the file served by modifying $uri_ref, and cancel the
 process by returning a true value.
 
+HANDLER     modify_response_headers     Perlbal::ClientHTTPBase
+Called when we've set all the headers, and are about to serve a file.
+You can change or add response headers at this point, or cancel the
+process by returning a true value.  You will have to send the response to
+the client yourself if you do this.
+
 HANDLER     backend_response_received   Perlbal::BackendHTTP
 Called as soon as response headers are read from the backend.  If you
 return a true value, will stop all handling at that point.

lib/Perlbal/ClientHTTPBase.pm

@@ -403,6 +403,8 @@ sub _serve_request {
 
             # has to happen after content-length is set to work:
             $self->setup_keepalive($res);
+
+            return if $self->{service}->run_hook('modify_response_headers', $self);
 
             if ($rm eq "HEAD" || $not_mod || $not_satisfiable) {
                 # we can return already, since we know the size