QtCore: fix the number precision in QJsonDocument.toJson()

In JSON, any number is stored in double. We need to make sure we
keep the maximum possible number precision for integer number. In
IEEE 754 double format, the significand precision is 53 bits(52
explicityly stored).

Autotest is included. qint64 and double work fine.

Task-number: QTBUG-28467
Change-Id: I7f857671c50e4334e9329c778f9b4f090f490540
Reviewed-by: Sune Vuorela <sune@vuorela.dk>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>