Fixes #2637 -- Clarified handling of TEMPLATE_STRING_IF_INVALID, especially with regards to filtering of invalid values. Modified unit tests to test both empty and non-empty values for TEMPLATE_STRING_IF_INVALID. Thanks to SmileyChris for identifying and helping to resolve this bug.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@3714 bcc190cf-cafb-0310-a4f2-bffc1f526a37