Find link in response body which href value matches ``href_pattern``.

        Returns found url or None.

def find_link(self, href_pattern, make_absolute=True):
        """
        Find link in response body which href value matches ``href_pattern``.

        Returns found url or None.
        """

        if make_absolute:
            self.tree.make_links_absolute(self.doc.url)

        if isinstance(href_pattern, six.text_type):
            raise GrabMisuseError('Method `find_link` accepts only '
                                  'byte-string argument')
        href_pattern = make_unicode(href_pattern)
        for elem, _, link, _ in self.tree.iterlinks():
            if elem.tag == 'a' and href_pattern in link:
                return link
        return None