Fix typo in ProxyHead().

It looks like there is just a typo in a variable name in the ProxyHead() function.

PhpStorm has some excellent features that other IDEs do not have that allows errors like these to be found. Other IDEs do not have said features and thus do not find said errors.
Merge pull request #2350 from vanilla/hotfix/proxyhead-port-fix

Fix typo in ProxyHead().
Merge pull request #2351 from vanilla/hotfix/device-override

Allow device types to overridden in the config.
Make sure the child iframe objects exist before sending a message.

This makes calling messages on a child iframe silently fail if the child window hasn’t loaded yet or loads the wrong url rather than potentially killing the page.
Merge branch 'hotfix/new-embed-sanity' into stage