Update mjcf schema

[hartikainen]

I noticed some differences between the mjcf xml schema and the documentation's XML Reference. This fixes two of these differences:

1. sensor/user element's attribute defaults. The corresponding change to mujoco was made in google-deepmind/mujoco@221c63d.
2. size element's memory attribute. I believe the corrensponding change to mujoco was made in google-deepmind/mujoco@58fd72f.

Also adds two test cases through test_model.xml, which both fail without these schema changes.