Allow mock Date object in isDate() check

[jamesisaac]

When carrying out automated testing (e.g. with Selenium), and wanting to test different timezones, it's common to mock the global Date object with a library like: https://github.com/plaa/TimeShift-js

Unfortunately, the isDate check in pickadate has a very strict check, which rejects anything that isn't a native JS Date object, rendering this mocking method unusable.

I propose making this check more lenient, so as long as the object looks like it matches the Date API, it's allowed through.

The issue manifests itself in an interesting way -- you end up with a calendar where, in the best case, every single date shows as today, and in the worst case, browser freezes up in an infinite loop.

[jamesisaac]

Not sure why Travis CI is failing - can't imagine how this would break things. Maybe to do with it being the last day of the month?

>> Message: Able to disable today
>> Actual: [
>>   2016,
>>   2,
>>   32
>> ]
>> Expected: [
>>   2016,
>>   3,
>>   1
>> ]

[DanielRuf]

It seems it was due to the overflow. You can check this with the current master on your machine by changing the system clock.

[DanielRuf]

Please rebase on the current head of master

[jamesisaac]

@DanielRuf I believe that's done now. I've also ticked "Allow edits from maintainers." in case you want to change something further yourself. Thanks.

[DanielRuf]

The changes probably cause the failing unit tests.

Can you investigate and test them?