cache issue with date_get_formats

Function date_get_formats should retrieve the user selected format for a date from the cache or figure out what the format is for a given field.

In the code (date.module,v 1.13.2.38) it fails to verify if the cache has that information and returns and empty format, thus when editing a view the filed help text reads:

"This filter allows events to be filtered by their date. Enter dates in the format: . Enter 'now' to ..."

when it shoud read:

"This filter allows events to be filtered by their date. Enter dates in the format: D Jan YYYY. Enter 'now' to ..."

The attached patch solves the issue.