Test for missing parameters

These currently fail, but shouldn't. Huzzah!
Check parameters in the correct place

Update doesn't require the user data, creation does.
Work around WP data handling failure
Check that we're not mangling user passwords
Merge pull request #289 from WP-API/flip-user-params-check

Flip user parameters check for insert/update.  Closes #289.  Fixes #221.