We could improve the list of known headers & values

[MihaZupan]

Some of the more common ones that we don't currently cache strings for:
(these were gathered from https://github.com/bensooter/URLchecker/blob/master/top-1000-websites.txt)

Connection: keep-alive
Connection: keep-alive, Transfer-Encoding

Cache-Control: private, max-age=0
Content-Length: 0
Vary: Accept-Encoding
Expires: -1
Age: 0

Accept-Ranges: bytes
Accept-Ranges: none

Content-Type: text/html; charset=utf-8
Content-Type: text/html
Content-Type: text/html;charset=utf-8

Strict-Transport-Security: max-age=31536000
Strict-Transport-Security: max-age=31536000; includeSubdomains
Strict-Transport-Security: max-age=31536000; includeSubdomains; preload

X-Frame-Options: SAMEORIGIN
X-Frame-Options: DENY

X-XSS-Protection: 1; mode=block
X-XSS-Protection: 0

X-Cache: HIT
X-Cache: MISS

X-Cache-Hits: 0
X-Cache-Hits: 1

We should also consider including names from the QPackStaticTable

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Some of the more common ones that we don't currently cache strings for:
(these were gathered from https://github.com/bensooter/URLchecker/blob/master/top-1000-websites.txt)

Connection: keep-alive
Connection: keep-alive, Transfer-Encoding

Cache-Control: private, max-age=0
Content-Length: 0
Vary: Accept-Encoding
Expires: -1
Age: 0

Accept-Ranges: bytes
Accept-Ranges: none

Content-Type: text/html; charset=utf-8
Content-Type: text/html
Content-Type: text/html;charset=utf-8

Strict-Transport-Security: max-age=31536000
Strict-Transport-Security: max-age=31536000; includeSubdomains
Strict-Transport-Security: max-age=31536000; includeSubdomains; preload

X-Frame-Options: SAMEORIGIN
X-Frame-Options: DENY

X-XSS-Protection: 1; mode=block
X-XSS-Protection: 0

X-Cache: HIT
X-Cache: MISS

X-Cache-Hits: 0
X-Cache-Hits: 1

Author:	MihaZupan
Assignees:	-
Labels:	area-System.Net.Http
Milestone:	-

[karelz]

Triage: We should ask ASP.NET (or Azure?) for most common headers. There is room for improvement here.

[halter73]

ASP.NET doesn't really optimize specific header values other than the ones Kestrel sets itself (e.g. Transfer-Encoding: chunked) and those in the static hpack/qpack tables. ASP.NET does optimize certain request and response header names. This is more of an art than a science though. No X- headers are optimized. It doesn't look like Strict-Transport-Security is optimized either. Maybe it should be.