Fix parsing bug for Firefox

Previously if you have model which was string that Firefox could not parse with `new Date` then you'd end up with an invalid date.

This change fixes that by parsing the date with provided `parseDateFunction`

A plunker of issue can be found at http://plnkr.co/edit/B12bHfRcJUgXrKcTDIlG?p=preview

Plus here is a plunker with the fix http://plnkr.co/edit/nudXIk6uym3lm1SnDmbw?p=preview

Note: can try to a test for this if needed, but seemed pretty hard to identify which part of the code the test should cover.