Get cache entry for key, or return None.

def get(self, orig_key):
        """Get cache entry for key, or return None."""
        resp = requests.Response()

        key = self._clean_key(orig_key)
        path = os.path.join(self.cache_dir, key)

        try:
            with open(path, 'rb') as f:
                # read lines one at a time
                while True:
                    line = f.readline().decode('utf8').strip('\r\n')
                    # set headers

                    if self.check_last_modified and re.search("last-modified", line, flags=re.I):
                        # line contains last modified header
                        head_resp = requests.head(orig_key)

                        try:
                            new_lm = head_resp.headers['last-modified']
                            old_lm = line[string.find(line, ':') + 1:].strip()
                            if old_lm != new_lm:
                                # last modified timestamps don't match, need to download again
                                return None
                        except KeyError:
                            # no last modified header present, so redownload
                            return None

                    header = self._header_re.match(line)
                    if header:
                        resp.headers[header.group(1)] = header.group(2)
                    else:
                        break
                # everything left is the real content
                resp._content = f.read()

            # status & encoding will be in headers, but are faked
            # need to split spaces out of status to get code (e.g. '200 OK')
            resp.status_code = int(resp.headers.pop('status').split(' ')[0])
            resp.encoding = resp.headers.pop('encoding')
            resp.url = resp.headers.get('content-location', orig_key)
            # TODO: resp.request = request
            return resp
        except IOError:
            return None