Build a dict with headers and values used in the signature.

        "signature_headers" is a list of lowercase header names.

def build_dict_to_sign(self, request, signature_headers):
        """Build a dict with headers and values used in the signature.

        "signature_headers" is a list of lowercase header names.
        """
        d = {}
        for header in signature_headers:
            if header == '(request-target)':
                continue
            d[header] = request.META.get(self.header_canonical(header))
        return d