Invalid serialization of undefined values in the middle of an object

[andyneville]

Using version 0.1.12-beta, as well as building the latest code locally from the master branch, serializing an object containing an undefined property sandwiched between other properties results in invalid JSON:

const value = {
  a: '123',
  b: undefined,
  c: '123'
};

console.log(JSON.stringify(value));

Expected output: {"a":"123","c":"123"} Actual output (LLRT): {"a":"123""c":"123"} which is invalid JSON because of the double double-quotes without a separating comma.

Moving the undefined property to the first or last position serializes properly:

./llrt -e "console.log(JSON.stringify({a: '123', b: undefined, c: '123'}));"
{"a":"123""c":"123"}    <- INVALID

./llrt -e "console.log(JSON.stringify({a: '123', c: '123', b: undefined}));"
{"a":"123","c":"123"}    <- valid

./llrt -e "console.log(JSON.stringify({b: undefined, a: '123', c: '123'}));"
{"a":"123","c":"123"}    <- valid