Why does the web server sent the file instead of a 304 http: not modified?

Question

My browser send to the server the following request:

Host: www.imprimante.be
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Firefox/38.0
Accept: */*
Accept-Language: fr,fr-FR;q=0.8,en-US;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
If-Modified-Since: Fri, 29 May 2015 14:22:44 GMT
If-None-Match: "90-5173935ad3a1a-gzip"
Referer: http://www.imprimante.be/premier-avis-gratuit/
Cookie: <hidden>
Connection: keep-alive

The url used is http://www.imprimante.be/wp-content/themes/mch_imprimante/js/theme.min.js? (note: www.imprimante.be is not accessible trough wlan yet)

And the server send me the file with this (status 200) http header:

Accept-Ranges: bytes
Connection: Keep-Alive
Content-Encoding: gzip
Content-Length: 137
Content-Type: application/javascript
Date: Wed, 03 Jun 2015 07:18:03 GMT
Etag: "90-5173935ad3a1a-gzip"
Keep-Alive: timeout=5, max=99
Last-Modified: Fri, 29 May 2015 14:22:44 GMT
Server: Apache/2.4.10 (Debian)
Vary: Accept-Encoding

As you might notice (Last-Modified: Fri, 29 May 2015 14:22:44 GMT) the file hasn't been modified since the last request.

So I don't get why the response isn't a 304 status: not modified.

I'd really like to know why the caching of this files (and some others) doesn't work as I expect it.

Answer 1

It is bug in Apache. Turn off mod_deflate.