add CORS check for 304 response

Some web servers, including it seems Apache in Fedora 22 do not set CORS headers on responses with a 304, e.g.:

$ curl -k -H 'If-None-Match: "1:c8ec8b42d2f03ce91d1ab7390ef56db5"' -I https://storage.tuxed.net/php-remote-storage/foo/public/shares/151019-1755-20150201_162437.jpg
HTTP/1.1 304 Not Modified
Date: Mon, 19 Oct 2015 18:13:03 GMT
Server: Apache/2.4.16 (Fedora) OpenSSL/1.0.1k-fips PHP/5.6.14
Etag: "1:c8ec8b42d2f03ce91d1ab7390ef56db5"

See https://bz.apache.org/bugzilla/show_bug.cgi?id=51223. Still not fixed.

[raucao]

Wow! That's been an issue since over 3 years now, and Apache still doesn't do CORS properly?

Yup, browsing the source, not even fixed in 'master': https://github.com/apache/httpd/blob/004c986675cfc468b85b487719bb4c6310b74afe/modules/http/http_filters.c#L1337

I guess I have to lie about the 304 and just return a 200 instead... :(

[raucao]

Damn, that's actually a really big drawback with all the directory requests we have to do for sync.

My tests show that 304 headers are not needed on CORS...

[raucao]

@fkooman What browsers/UAs did you test so far? We should probably make a list, as this was a problem with all browsers I tested not too long ago.

[fkooman@localhost ~]$ rpm -q firefox chromium
firefox-42.0-2.fc22.x86_64
chromium-46.0.2490.80-1.fc22.x86_64
[fkooman@localhost ~]$