Bug 13011: 0000-00-00 should not be considered as a valid date

Bug 13011: 0000-00-00 should not be considered as a valid date

When parsing the letter content, the 0000-00-00 should not be replaced,
it's not a valid date.

Test plan:
  prove t/db_dependent/Suggestions.t
should not return the following error:
0000-00-00 seems to be a date but an error occurs on generating it (The
'month' parameter ("0") to DateTime::new did not pass the 'an integer
between 1 and 12' callback

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Adds another check for 0000-00-00.
Passes tests and QA script.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>