Avoid init-time exception for applewebdata: based URL found in native platform WebView

This affects apps that use TinyMCE embedded into a WebKit view in a native app where the content is not file based. In this scenario the base URL for a document is a custom Apple scheme like "applewebdata://5B6058BF-649C-431B-BF28-358D68108A9E" and the default behavior of TinyMCE strips this to just "applewebdata:/" which causes errors later in URI processing. 

The patch provided will special case applewebdata: URLs leaving them as-is, which I've tested over a couple years at least with TinyMCE3 and it seems to work well.

Unfortunately I couldn't see an obvious way to add a unit test for this scenario. The behavior without this patch is if you were to try loading TinyMCE into a WebKit based view on Mac or iOS, it will throw an exception in initializing EditorManager, and thus the whole load of TinyMCE ends up failing.