[1.2.X] Fixed #13668 -- Corrected database router methods invocation for...

[1.2.X] Fixed #13668 -- Corrected database router methods invocation for ManyToMany fields without through models. Thanks craig.kimerer for the report and David Gouldin for the fix.

This also adds tests for r14857.

Backport of [15185] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15186 bcc190cf-cafb-0310-a4f2-bffc1f526a37