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
Merge pull request #2708 from chillu/pulls/addsearchparams

Merge query strings in $.path rather than duplicating