Returns a list of headers fields to sign.

        According to http://tools.ietf.org/html/draft-cavage-http-signatures-03
        section 2.1.3, the headers are optional. If not specified, the single
        value of "Date" must be used.

def get_headers_from_signature(self, signature):
        """Returns a list of headers fields to sign.

        According to http://tools.ietf.org/html/draft-cavage-http-signatures-03
        section 2.1.3, the headers are optional. If not specified, the single
        value of "Date" must be used.
        """
        match = self.SIGNATURE_HEADERS_RE.search(signature)
        if not match:
            return ['date']
        headers_string = match.group(1)
        return headers_string.split()