Add 'always_send' option to always send p3p header

Allow forcing the P3P to always send, regardless of user-agent-sniffing check.

Originally arose because IE11 changes their user-agent to NOT include "MSIE" like prev browsers.

http://blogs.msdn.com/b/ieinternals/archive/2013/09/21/internet-explorer-11-user-agent-string-ua-string-sniffing-compatibility-with-gecko-webkit.aspx

IE11: `Mozilla/5.0 (Windows NT 6.3; Trident/7.0; rv:11.0) like Gecko`
IE10: `Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Trident/6.0)`