[#1117] Add new tests for updating user names and passwords

...and also for what happens when you call user_update without a valid
id.
[#1117] user_update don't crash on non-string name

Don't crash if the user tries to update their user name to a value that
is not a string (which they can do, using the API)
[#1117] user_update: don't crash on non-string password

Fix a crash that happens if the user tries to update their password to a
value that isn't a string (which they can do, using the API)