function unionClassNames(existingClassNames, additionalClassNames) {
  if (!existingClassNames && !additionalClassNames) return '';
  if (!existingClassNames) return additionalClassNames;
  if (!additionalClassNames) return existingClassNames;
  return union(existingClassNames.split(' '), additionalClassNames.split(' ')).join(' ');
}
Returns a string containing all classes without duplicates.

@param existingClassNames {String} - one or multiple classes
@param additionalClassNames {String} - one or multiple classes

@example
// returns 'style-id-23 button buy-button'
unionClassNames('style-id-23 button', 'button buy-button')

Originally inspired by https://github.com/rackt/react-autocomplete/blob/master/lib/union-class-names.js