public function call() {
		$resp = $this->app->response;
		foreach ( $this->headers as $name => $value ) {
			if ( is_callable( $value ) ) {
				$value = $value( $this->app );
			}

			if ( $value !== null ) {
				$resp->header( $name, $value );
			}
		}

		$this->next->call();
	}