Undefined index HTTP_USER_AGENT warning with PHP 5.4

[Google Code Exporter]

Minify commit/version: 2.1.5
PHP version: 5.4

What steps will reproduce the problem?
1. Use Minify or use a WordPress plugin utilizing this Minify.

Expected output:
No warning

Actual output:
Notice: Undefined index: HTTP_USER_AGENT in /xxxx/min/lib/HTTP/Encoder.php on 
line 321

Original issue reported on code.google.com by [e-mail] on 29 Jul 2013 at 1:57