CSRF fix for Ajax POST mentioned in docs intermittently fails to append token for IE7

[nick@…]

I've found that in some cases(not sure why), IE7 will prepend protocol://servername to a form's action, causing the

 if (!(/^http:.*/.test(settings.url) || /^https:.*/.test(settings.url))) {...}

test to fail...

I propose we use the following instead, as it will work in more cases:

    var page_host = window.location.host;
    var regex=new RegExp('^https?://' + page_host + '/', 'i');
    if (regex.test(settings.url) || !(/^http:.*/.test(settings.url) || /^https:.*/.test(settings.url))) {
        // Only send the token to relative URLs i.e. locally.
    }

[Alex Gaynor]

Dupe of #15869