code: function _getDigestAuthHeader (headers) {
      return headers.find(function (property) {
          return (property.key.toLowerCase() === WWW_AUTHENTICATE) && (_.startsWith(property.value, DIGEST_PREFIX));
      });
  }
docs: Returns the 'www-authenticate' header for Digest auth. Since a server can suport more than more auth-scheme,
  there can be more than one header with the same key. So need to loop over and check each one.
  
  @param {VariableList} headers
  @private