hard to debug when fpassthru is disabled

[GoogleCodeExporter]

What steps will reproduce the problem?
1. On apache, accessing a minify url properly generate the content and save the 
cache as expected.
2. On a second acess to the same url, the cached version is accessed but 
fpassthru function fails because of a system configuration wich "disables 
fpassthru for security reasons".

Expected output:
cached content being passed through

Actual output:
The output is truncated, the browser shows the message "this web page is 
unavailable". 

This was particularly hard to debug since there's no readable message, not even 
on an HTTP debugger. I guess this is related to the gzip headers being sent 
before the fpassthru. I managed to use file_get_contents to read the url and 
finally get the message wich is 
Warning: fpassthru() has been disabled for security reasons in 
/home/XXX/www/min/lib/Minify/Cache/File.php on line 84

I'm aware this is not a minify issue per se, but it would be nice to add a unit 
test for that function. 

Original issue reported on code.google.com by claudiof...@gmail.com on 23 Oct 2013 at 10:20