[#153] PUSH_PROMISE headers

[Martin Thomson <[e-mail]>]

https://github.com/http2/http2-spec/issues/153

The current text describes PUSH_PROMISE as having a few request
headers, plus some response headers, but it's quite vague.

I think that if this is going to be properly workable across a wide
range of uses with lots of different headers, PUSH_PROMISE needs to
include two sets of headers: the ones that it overrides from the
associated request (:path being foremost of those) and the ones that
it provides as a "preview" of the response (e.g., ETag might allow
caches to determine if they were interested in the rest of the
response).