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##### websocket::stream::async_pong

Start an asynchronous operation to send a WebSocket pong frame.

###### Synopsis

    template<
        class WriteHandler>
    DEDUCED
    async_pong(
        ping_data const& payload,
        WriteHandler&& handler);

###### Description

This function is used to asynchronously send a pong frame to the stream. The function call always returns immediately. The asynchronous operation will continue until one of the following is true:

- The entire pong frame is sent.
- An error occurs on the stream.

This operation is implemented in terms of one or more calls to the next layer's `async_write_some` functions, and is known as a composed operation. The program must ensure that the stream performs no other writes until this operation completes.

The WebSocket protocol allows pong frames to be sent from either end at any time. It is not necessary to first receive a ping in order to send a pong. The remote peer may use the receipt of a pong frame as an indication that the connection is not dead.

If a close frame is sent or received before the pong frame is sent, the completion handler will be called with the error set to `boost::asio::error::operation_aborted`.

###### Parameters

Name | Description
--- | ---
`payload` | The payload of the pong message, which may be empty.
`handler` | Invoked when the operation completes. The handler may be moved or copied as needed. The function signature of the handler must be:
    void handler(
        error_code const& ec     // Result of operation
    );
Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using `boost::asio::io_context::post`.