Reject invalid user attributes on assignment

Invalid user attributes cause XML that the server cannot process, so they should be rejected on assignment, in addition to the warning that is already provided.

I'd need some verification from Airbrake itself first to verify that invalid user attributes do actually cause malformed XML. I had a project where I was passing an invalid user attribute (full_name) and it caused the errors to just hang in the UI waiting to be processed. Once I removed that attribute, the errors processed and appeared fine.