Validation BUG in update_user #117

In update_user there is no isset checks for $userdata['username'] and $userdata['email'].

Which means that to be able to update the user profile using this plugin, we always need to send username and email, otherwise we get PHP errors.

For example:

exception | "Whoops\\Exception\\ErrorException"
message | 'Undefined array key "username"'

Is this intentional, or a BUG?

I think it would be nice if only the field that we want to update is a requirement.

Anyhow, I solved this for our integration. And can submit a PR during the day (since we are in different timezones, I hope it's okay not to wait)