Fixed #20401 - ContentTypeManager.get_for_model reads from db_for_read
Fixed the fix for #20401 - Added a forgotten .save()
Added test for a special case of ContentTypeManager.get_for_model()

If ContentTypeManager.get_for_model() doesn't find in the database a row
for a model, it adds it; this occurs very rarely, if ever. This commit
adds a test for that case.