[FIX] When creating a new user, "Custom Fields" are breaking functionality

[dnlsilva]

Proposed changes

Send custom fields after user registration, in the same way as the front end.

Issue(s)

How to test or reproduce

Try to register a user on a server that has custom fields on the registration screen.

Example: mobile server

Tested on:
5.2
5.0
4.8

Screenshots

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves a current function)
• New feature (non-breaking change which adds functionality)
• Documentation update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• I have added necessary documentation (if applicable)
• Any dependent changes have been merged and published in downstream modules

Further comments