Description:
------------
In case of zlib.output_compression turned on and if the script's only output is HTTP header "304 Not Modified", actual PHP output is 20 bytes of gzipped nothing (ungzipping this data produces empty stream) without "Content-Length" header set.

Example (see zlib-test.php below):

~> curl --compressed -v http://myhost.com/zlib-test.php
> GET /zlib-test.php HTTP/1.1
> Host: myhost.com
> User-Agent: curl/7.54.0
> Accept: */*
> Accept-Encoding: deflate, gzip
> 
< HTTP/1.1 304 Not Modified
< Date: Sat, 02 Sep 2017 09:54:26 GMT
< Connection: keep-alive
< Vary: Accept-Encoding
< 
* Excess found in a non pipelined read: excess = 20 url = /zlib-test.php (zero-length body)

Excessive 20 bytes could be a problem on client side if keep-alive is used: since Content-Length is unknown, the data will stay in buffer until next request and will be read as the beginning of the response.

The issue persists between PHP version (tested with 7.0.21 and 7.0.23) and regardless of webserver setup (Apache + mod_php or nginx + PHP-FPM).

Test script:
---------------
<?php
ini_set("zlib.output_compression", 4096);
header("HTTP/1.1 304 Not Modified");