Correctly encode the audit log header to a URI component

[Rapougnac]

Description

Audit log reason should now correctly handle UTF-8 strings.

Tests are still missing, I'm planning to do them tomorrow

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] Ran dart analyze or make analyze and fixed all issues
• [x] Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works