Unable to assert invalid dates

[meandmycode]

When asserting something that may return an invalid date mocha will error trying to report the invalid date due to calling toISOString on an invalid date.

Example (using should):

describe('mocha', function() {
  it('should not error when comparing invalid dates', function() {
    new Date('').should.equal(new Date(''));
  })
})

I can submit a PR if it helps, I think the stringify utility just needs to call toString() in the event that the date is invalid, which will result in "[Date: Invalid Date]"

[a8m]

Thanks @meandmycode
PTAL on #1661

[meandmycode]

Much appreciated, speedy work!

[boneskull]

closed via #1661