http2session.alpnProtocol
如果 Http2Session 尚未连接到套接字，则值为 undefined，如果 Http2Session 未连接到 TLSSocket，则值为 h2c，或者将返回已连接的 TLSSocket 自己的 alpnProtocol 属性的值。
Value will be undefined if the Http2Session is not yet connected to a
socket, h2c if the Http2Session is not connected to a TLSSocket, or
will return the value of the connected TLSSocket's own alpnProtocol
property.