fix: handle _redirects for If-None-Match headers

[aschmahmann]

Fixes a bug where If-None-Match requests would return 404s when _redirects files should've been used.

[codecov]

Codecov Report

Merging #412 (d9b38ab) into main (f6b448b) will increase coverage by 0.13%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             main     #412      +/-   ##
==========================================
+ Coverage   49.62%   49.76%   +0.13%     
==========================================
  Files         248      248              
  Lines       29912    29933      +21     
==========================================
+ Hits        14843    14895      +52     
+ Misses      13640    13610      -30     
+ Partials     1429     1428       -1     

Files Changed	Coverage Δ
gateway/handler_defaults.go	37.65% <0.00%> (-2.62%)	⬇️
gateway/handler.go	78.91% <58.33%> (+0.98%)	⬆️

... and 10 files with indirect coverage changes

[aschmahmann]

@lidel HEAD requests currently skip _redirects, I forget some of our prior conversation on if they should respect them as well.

[lidel]

@aschmahmann according to HTTP specs the behavior for HEAD should be the same as GET,
but it has such niche utility, that we said it is ok to skip the _redirect logic for HEAD if it is too much work in MVP (#176).

iiuc next steps here:

• see if we can make HEAD the same as GET while we are at it
• add regression test described in ipfs/gateway-conformance#111

[BigLep]

2023-07-20 conversation: this will make it in Kubo 0.22. We're not planning to do a Kubo 0.21 patch release currently with this. This was reported by Fleek.

@aschmahmann will create an issue for this and notify #ipfs-operators in case anyone wants to block the header for the time being.

[hacdias]

LGTM, added a test. Pinging @lidel for a second look at the test.

[lidel]

lgtm, thank you!