Issue #5214 - Use known content_length when in bypass write

+ In the case of HEAD, the servlet-api response is a wrapper
  of javax.servlet.http.HttpServlet$NoBodyResponse
  We know the content_length, use it.

Signed-off-by: Joakim Erdfelt <joakim.erdfelt@gmail.com>

jetty-server/src/main/java/org/eclipse/jetty/server/ResourceService.java

@@ -677,7 +677,7 @@ protected boolean sendData(HttpServletRequest request,
             else if (written || !(out instanceof HttpOutput))
             {
                 // write normally
-                putHeaders(response, content, written ? -1 : 0);
+                putHeaders(response, content, content_length);
                 ByteBuffer buffer = content.getIndirectBuffer();
                 if (buffer != null)
                     BufferUtil.writeTo(buffer, out);