Fix the problem with chunked encoding with an empty body.

When the response body is empty, rocket won't send the final zero-length
terminating chunk for chunked encoding. I think this causes the browser
to wait for the connection close in order to tell the end of the
response.

gluon/rocket.py

@@ -1850,12 +1850,13 @@ class WSGIWorker(Worker):
                 if data:
                     self.write(data, sections)
 
+            if not self.headers_sent:
+                # Send headers if the body was empty
+                self.send_headers('', sections)
+
             if self.chunked:
                 # If chunked, send our final chunk length
                 self.conn.sendall(b('0\r\n\r\n'))
-            elif not self.headers_sent:
-                # Send headers if the body was empty
-                self.send_headers('', sections)
 
         # Don't capture exceptions here.  The Worker class handles
         # them appropriately.