Cannot set Auth Source to Local via API

[leesei]

Describe the bug
Cannot set Auth Source to Local via API

To Reproduce
Steps to reproduce the behavior:

1. Create an Authentication Source (say a SMTP auth source with id 2)
2. Change user to the new auth source in admin panel
3. Change auth source to local (id 0) with API
   http --print HhB PATCH http://${GOGS_HOST}/api/v1/admin/users/$GOGS_USER "Authorization: token ${GOGS_TOKEN}" source_id:=0 login_name=$GOGS_USER email=$GOGS_USER@mail.com password=$GOGS_USER
4. Server response 200 but auth source not changed (per admin panel and login error message)

Expected behavior
The auth source should be set to Local after the 200 response

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context

• Gogs version (or commit ref): 0.11.86.0130
• Git version:
• Operating system: Linux
• Database (replace [ ] with [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gogs.io:
  • Yes: provide example URL
  • No: explain why
    no external auth source
• Log gist (usually found in log/gogs.log):

[unknwon]

I think the cause is the zero-value feature of Go. 0 itself is not recognized as a change of state.