Improved isDate() method

`isDate()` does not check for the weird `Invalid Date` 

    var date = new Date('not a date');  //  Invalid Date
    _.isDate(date);  //  true

Now, also testing with `!isNaN(date.getTime())`

    _.isDate = function(obj) {
      return toString.call(obj) === '[object Date]' && !isNaN(obj.getTime());
    }; 

Result:

    _.isDate(date);  //  false