HTTP2 server-side stream creation

Hello!

During development of [spdy-transport][0] library for node.js, one
of the contributors asked about the server-side stream creation in
HTTP2 protocol.

Obviously, the most straightforward way is to do a PUSH_PROMISE on
existing client-initiated stream, but it appears to me that the
server-initiated streams created using HEADERS frame are valid
too.

Is there any place in spec that prohibits it?

Thank you very much for your time,
Fedor.

[0]: https://github.com/indutny/spdy-transport <https://github.com/indutny/spdy-transport>
[1]: https://github.com/indutny/spdy-transport/issues/6 <https://github.com/indutny/spdy-transport/issues/6>