Support for Node.js 8 native http2

Sorry for opening another issue on this topic (#77, #78), but I'm trying out the new native http2 support in Node.js and compression (1.7.1) fails with:

````
TypeError: this._implicitHeader is not a function
at Http2ServerResponse.write (node_modules/compression/index.js:84:14)
````

As far as I see it, this method isn't part of the public API of [http.ServerResponse](https://nodejs.org/api/http.html#http_class_http_serverresponse). Shouldn't compression simply call `if (!this.headersSent) this.writeHead(this.statusCode)` (in a helper function) instead of depending on undocumented API?