Merge query strings in $.path rather than duplicating

Fixes common duplications like ?locale=en_US&locale=en_US, triggered e.g. by
gridfield's showDetailView().

See https://github.com/silverstripe/silverstripe-translatable/issues/154

This modifies code in a third party lib, but `jQuery.path` has been extracted from the jQuery mobile project anyway so its not easily upgradeable. I've had a quick look though, the search param logic is still the same one in current jQuery mobile.