JSON: Fix MSVC2013 warnings

json.cpp(221) : warning C4800: 'uint64_t' : forcing value to bool 'true' or 'false' (performance warning)
json.cpp(322) : warning C4800: 'uint32_t' : forcing value to bool 'true' or 'false' (performance warning)
json.cpp(756) : warning C4244: '=' : conversion from 'int64_t' to 'double', possible loss of data
json.cpp(953) : warning C4244: 'return' : conversion from 'const double' to 'int', possible loss of data
json.cpp(4408) : warning C4244: 'argument' : conversion from '__int64' to 'int', possible loss of data

Change-Id: I2a24f90f7615aeb47f747ecbe3b580f23773ebda
Reviewed-by: hjk <hjk@theqtcompany.com>