This PR fixes issue #377, by relying on current_time() instead of date_format() to properly produce a default value for user_registered when creating users via wp user create.

I've also included a behat test that should fail in master (but not on this branch).

Note: I did a quick search through the org and this seems to be the only place where we were using date_format() with an invalid format.

Steps to test

1. Check out master.
2. Run wp user create testuser testuser@example.com.
3. Check that wp user get testuser shows 0000-00-00 00:00:00 as the value for the user_registered field.
4. Delete the user created in step 2. For example, with wp user delete testuser.
5. Check out this branch.
6. Repeat step 2 and confirm that this time the current date/time is the value of the user_registered field.

Closes #377.