Allow WithTransform()'s function to accept a nil value

[ghost]

Add a unit test to verify this use case.

[onsi]

awesome, thanks!

[cbandy]

This is no longer checking that the actual = nil is appropriate for the transform function. This incorrectly succeeds:

gomega.WithTransform(
	func(int) bool { return true },
	gomega.BeTrue(),
).Match(nil)

I recommend the following:

	var param reflect.Value
	if actual != nil && reflect.TypeOf(actual).AssignableTo(m.transformArgType) {
		// The original type of actual is compatible with the transform argument.
		param = reflect.ValueOf(actual)

	} else if actual == nil && m.transformArgType.Kind() == reflect.Interface {
		// The original type of actual is unknown, so there's no way to make its
		// reflect.Value. Create a nil of the transform argument, which is known.
		param = reflect.Zero(m.transformArgType)

	} else {
		return false, fmt.Errorf("Transform function expects '%s' but we have '%T'", m.transformArgType, actual)
	}

[onsi]

thanks @cbandy - are you up for submitting this as a PR?

[cbandy]

Yep, no problem. Submitted #423.