[Creatable] Fix multiple clicks required to create new option

[billyvg]

When you attempt to create a new option, it does not have a value property -- the "Create new label" option also does not have a value property, so isOptionUnique believes they are equal.

Is this the right place to handle the issue?

Fixes #1327

[coveralls]

Coverage remained the same at 92.308% when pulling 4a0fc79 on billyvg:master into ef3a468 on JedWatson:master.

[JedWatson]

Can you provide a full usage example here? I can't reproduce the behaviour you're seeing.

I suspect you're using a custom newOptionCreator method that's not setting the value property on the new option, the default one sets both the value and the label property to the input value:

function newOptionCreator ({ label, labelKey, valueKey }) {
	const option = {};
	option[valueKey] = label;
 	option[labelKey] = label;
 	option.className = 'Select-create-option-placeholder';
 	return option;
};

[billyvg]

That's correct, we have an option whose value as an empty string.

[coveralls]

Coverage increased (+0.4%) to 92.707% when pulling 90ea526 on billyvg:master into ef3a468 on JedWatson:master.