http2.getDefaultSettings()
返回包含 Http2Session 实例默认设置的对象。
此方法每次调用时都会返回新的对象实例，因此可以安全地修改返回的实例以供使用。
- Returns: <HTTP/2 Settings Object>
Returns an object containing the default settings for an Http2Session
instance. This method returns a new object instance every time it is called
so instances returned may be safely modified for use.