JsonPrinter remove trailing garbage after non-printable char

[ywhrh]

SBE toString() won't print trailing garbage after non-printable char; JsonPrinter should have the same functionality...

see CarEncoder toString:

for (int i = 0; i < vehicleCodeLength() && this.vehicleCode(i) > 0; i++)
{
    builder.append((char)this.vehicleCode(i));
}