Unable to pass `undefined` as mock return value

[steevsachs]

Currently an Env mapping is missing a value for... error will be thrown when a mapping specifies undefined as its return value, ie

[call(shouldReturnUndefined), undefined]

due to the check on line 164 of core.js:

var nextValFound = nextVal !== undefined;

I believe undefined is a valid return value, so this throw condition seems to be in error. The absence of a return value should instead be caught by isNestedArray, which confirms the array has a length of 2.

My suggested fix would be to remove line 164 (happy to do a PR!), but raising an issue to confirm there's not some other reasoning behind this assertion.

[timbuckley]

@steevsachs I think that's a fair point. Happy to receive a PR on it! :-)