Fixes #1338, Refs #1400, #2237 -- Modified variable resolution to allow...

Fixes #1338, Refs #1400, #2237 -- Modified variable resolution to allow template 'if' statements to work if TEMPLATE_STRING_IF_INVALID is set. Modified unit tests to force the use of this variable, so that returning '' isn't confused with an actual failure.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@3268 bcc190cf-cafb-0310-a4f2-bffc1f526a37