Value of json expression is `undefined`

[kennethklee]

When running this, value, _ = vm.Run("{}"), the resultant value is undefined instead of an Object.

Other types work:

value, _ = vm.Run("1")
// value.toString() is 1

value, _ = vm.Run("true")
// value.toString() is true

value, _ = vm.Run("[1, 2, 3]")
// value.toString() is 1, 2, 3

As an extension, an error is returned when using { "a": 1 }. Error: (anonymous): Line 1:5 Unexpected token :

[kennethklee]

I think this is related to #430