XML: Refactor of feature detecting logic, resolves #1435

[ezequiel]

This fixes #1435.

Because of better feature detection, IE 9, 10, and 11 now use DOMParser whenever possible, rather than relying on ActiveXObject first.

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Before, Y.XML.parse relied on the browser throwing a ReferenceError when ActiveXObject did not exist. If, and only if that error was thrown, it would then fall back to DOMParser.

IE 11 has now set ActiveXObject to undefined. So,

// normally throws a `ReferenceError` if the variable doesn't exist, but *doesn't* in IE 11.
ActiveXObject; // undefined;

typeof ActiveXObject; // undefined

"ActiveXObject" in window; // true

[ezequiel]

They both do exactly the same thing. As Y.Lang.isNull did not provide any benefits, I removed all traces of it.