Use proper headers for cache expiration

[idan]

X-Expire-TTL-Seconds on gistio.py:47 is simply retarded.

[Potherca]

@idan ✔️ Issue can be closed.

The response headers no longer contain a X-Expire-TTL-Seconds header:

Key	Value
access-control-allow-origin	*
cache-control	s-maxage=0
content-disposition	inline; filename="index.html"
content-encoding	br
content-type	text/html; charset=utf-8
date: Sat, 01 Jun 2019 13:08	44 GMT
etag	W/"d5764eb602a507d61ee782f6a8590c80aaa7cfc609978425c57b882a843c4e0b"
server	now
status	200
strict-transport-security	max-age=63072000
x-now-cache	HIT
x-now-id: bru1	538jj-1559394524531-bb0bcf3d7bdc
x-now-trace	bru1