code: function(path, queryParams, prefix) {
          let queryString = "";
          if (queryParams) {
              queryString = "?" + utils.encodeParams(queryParams);
          }
          return this.opts.baseUrl + prefix + path + queryString;
      }
docs: Form and return a homeserver request URL based on the given path
  params and prefix.
  @param {string} path The HTTP path <b>after</b> the supplied prefix e.g.
  "/createRoom".
  @param {Object} queryParams A dict of query params (these will NOT be
  urlencoded).
  @param {string} prefix The full prefix to use e.g.
  "/_matrix/client/v2_alpha".
  @return {string} URL