More flexible date handling in date mapper

While writing #552480: More extensive tests in feedapi_mapper_date.test, I realized that the following mapping does not work:

Wed, 08/19/2009 - 12:16 -> start_date

It tries rigidly to append the start_time to the end of the source string, so the date parsing (date_create) will fail.
I'd say it's not user-friendly enough. I changed this behaviour.
Also i replaced an isset to an is_object to avoid warnings if somehow the date parsing really fails.