Add defensive coding to support experimental object spread properties

[Morantron]

In a file like this with experimental object spread properties syntax:

function lol(a, b, c) {
  return {...a, b, c};
}

The following rules were throwing an error because node.property is undefined in a property node of type ExperimentalSpreadProperty.

Not sure if this is the right approach, but at least it allows me to run eslint!

[yannickcr]

Thanks for the fix!