add basic 503 retry-after with spec

[hthetiot]

http2.js issue:

• https://github.com/kaazing/http2.js/pull/20

http2-cache.js build using http2.js#features/retry-afer

• https://gist.github.com/hthetiot/9bc89a3c19ca4e7ea9d159754b6bf298

Reference:

• https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.37
• https://stackoverflow.com/questions/3764075/retry-after-http-response-header-does-it-affect-anything
• https://social.msdn.microsoft.com/Forums/ie/fr-FR/81a0c3c3-aee3-4103-9b1c-731910903514/http-error-503-retryafter-header-value-format?forum=os_exchangeprotocols

[hthetiot]

Checking why spec fail here by pass in http2.js now

• https://github.com/kaazing/http2.js/pull/20#issuecomment-404967572

[hthetiot]

I will look why test fail and if related to gzip "fix" on open half close.

Here is a fresh http2-cache.js build with http2.js retry-after support builtin for testing:

• https://gist.github.com/hthetiot/9bc89a3c19ca4e7ea9d159754b6bf298 (2 files full and min)

[dpwspoon]

LGTM, other than the test failure