JSON: Replace `value.hasOwnProperty` with `Object.prototype.hasOwnProperty.call`

[robinheghan]

Replace value.hasOwnProperty with Object.prototype.hasOwnProperty.call in the JSON module

core/src/Gren/Kernel/Json.js

Line 234 in 16aa6c5

if (value.hasOwnProperty(key)) {

This makes it harder for someone to use hasOwnProperty for something other than it's intended purpose.