docs: 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()