admin: class-frontend fix a long-running x-domain tracking quoting bug [1]

Serious breakage involved, capable of completely crippling page functionality in some browsers. This has been discussed on the plugin support forum for 2 years, yet nobody has been able to send a patch upstream. So here you go.

[1]: http://wordpress.org/support/topic/plugin-google-analytics-for-wordpress-cross-domain-tracking