function keys( value ) {
	if ( isArguments( value ) ) {
		return builtin( slice.call( value ) );
	}
	return builtin( value );
}
Returns an array of an object's own enumerable property names.

@private
@param {*} value - input object
@returns {Array} a list of own enumerable property names

@example
var obj = {
    'beep': 'boop',
    'foo': 3.14
};

var k = keys( obj );
// e.g., returns [ 'beep', 'foo' ]