Proxy throws E_NOTICE errors

[giles-v]

Steps to reproduce:

• Set up PHP to error_reporting E_ALL
• Use the proxy to request a page (not using 'send_session' or 'user_agent' GET params)
• Observe output

Expected result:

• Output will be free of PHP errors

Actual result:

• E_NOTICE errors are thrown from lines 172 and 184, and pollute the response.

Suggested fix: use "if (!empty($var))" instead of "if ($var)" to avoid throwing errors if the variables are undefined. There are probably a few more of these in the code too, these were the only I observed with my settings.