Avoid incurring the cost of converting the latin1 data in that case.

Several existing QString unit tests excercise the new code path.

Task-number: QTBUG-52617
Change-Id: I27256d9e7db34f09543e244a79d754ff7932f0d0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>