Registering an anomymous user does not clear `authData.anonymous`

[acinader]

At least the JS SDK and the IOS SDK clear anonymous auth data when a user registers.

Here's the js unit test: https://github.com/parse-community/Parse-SDK-JS/blob/master/src/__tests__/ParseUser-test.js#L675

The php sdk does not do this.

[acinader]

fyi, the fix #417 mimics the logic in JS SDK https://github.com/parse-community/Parse-SDK-JS/blob/db59b63bcc37abf914a8b4fb261354e8aa6cdcc4/src/ParseUser.js#L285