Avoid spurious failures in naturaltime tests.
Avoid spurious failures in naturaltime tests.

Previous fix didn't cover timesince module, which naturaltime delegates to
under certain conditions. Extended mock of datetime to timesince module, and
switched to a fixed datetime rather than now() for testing, for more
reproducible tests (failures in mock will fail fast, rather than sporadically).

git-svn-id: 554f83ef17aa7291f84efa897c1acfc5d0035373@17038 bcc190cf-cafb-0310-a4f2-bffc1f526a37

svn2github/django,svn2github/django,svn2github/django