Add new to DateTimeFormat invocation

[TrevorSayre]

In our project we mock Intl.DateTimeFormat in order to quickly run tests against various timezones. I understand your preferred method differs, but this had been working for us up to version 1.1.4 quite nicely.

In 1.1.5 a commit introduced isValidTimezoneIANAString which invokes Intl.DateTimeFormat without new. When mocking, this causes the error Uncaught TypeError: Class constructor DateTimeFormatFake cannot be invoked without 'new' which subsequently causes an Invalid Date error. The fix is as simple as adding new, which shouldn't affect anyone who isn't mocking, but may help those who would like to.

[marnusw]

Thank you. I'll sort this later today. If you'd like to contribute an example of how you run your tests to the docs that would be much appreciated as it sounds like something others might benefit from.

[TrevorSayre]

Thank you! I'll look into writing up an example of how we do our jest tests across multiple timezones.

[marnusw]

1.3.4