code: def finish(self):
          
          stylesheet = ''.join(self._buffer)
          parser = CSSParser()
          css = parser.parseString(stylesheet)
          replaceUrls(css, self._replace)
          self.request.write(css.cssText)
          return self.request.finish()
docs: Parse the buffered response body, rewrite its URLs, write the result to
          the wrapped request, and finish the wrapped request.