http2 support relies on deprecated function

[vladmandic]

Enabling http2 in Redbird causes runtime warning:

(node:56863) [DEP0066] DeprecationWarning: OutgoingMessage.prototype._headers is deprecated
at ServerResponse.writeHead (/home/vlado/dev/piproxy/node_modules/spdy/lib/spdy/response.js:18:12)
at ServerResponse._implicitHeader (http_server.js:240:8)
at write (_http_outgoing.js:660:9)
at ServerResponse.write (_http_outgoing.js:636:15)
at IncomingMessage.ondata (_stream_readable.js:712:22)
at IncomingMessage.emit (events.js:315:20)
at IncomingMessage.Readable.read (_stream_readable.js:512:10)
at flow (stream_readable.js:985:34)
at resume (_stream_readable.js:966:3)
at processTicksAndRejections (internal/process/task_queues.js:84:21)

Which is caused by a known spdy issue:
[(https://github.com/spdy-http2/node-spdy/issues/366)]

given that nodejs nowadays supports http2 natively, I think depencency on spdy is no longer needed.

[vladmandic]

closing as no update from maintainer for over a year.