function isIntegerString( prop ) {
	return ( isString( prop ) && RE_INTEGER.test( prop ) );
}
Tests if an indexing expression is an integer.

@private
@param {(string|symbol)} prop - property name
@returns {boolean} result

@example
var out = isIntegerString( '1' );
// returns true

@example
var out = isIntegerString( ':' );
// returns false