http2stream.pushAllowed
只读属性映射到远程客户端最近的 SETTINGS 帧的 SETTINGS_ENABLE_PUSH 标志。
如果远程节点接受推送流，则为 true，否则为 false。
同一个 Http2Session 中的每个 Http2Stream 的设置都是相同的。
Read-only property mapped to the SETTINGS_ENABLE_PUSH flag of the remote
client's most recent SETTINGS frame. Will be true if the remote peer
accepts push streams, false otherwise. Settings are the same for every
Http2Stream in the same Http2Session.