Added failing test cases for giving a DateField or DateTimeField a None value to serialize.

Signed-off-by: Kevin Stone <kevinastone@gmail.com>
Patched DateField and DateTimeField to check for None values before trying to perform date(time) conversion.

Signed-off-by: Kevin Stone <kevinastone@gmail.com>
Merge pull request #708 from kevinastone/master

Fixed DateField with Empty Values (Fixes #707)