setHeaders(uri, headers) {
    // Copy the header over to a map and then back to an object. This makes
    // a copy of the original header.
    const map = shaka.util.MapUtils.asMap(headers);
    this.headersMap_.set(uri, shaka.util.MapUtils.asObject(map));
    return this;
  }
Sets the headers for a specific uri.

@param {string} uri
@param {!Object<string, string>} headers
@return {!shaka.test.FakeNetworkingEngine}