fix(search): add charset=utf-8 to headers of indexed mode

[HADB]

🔗 Linked issue

#2728

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation or readme)
• [x] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [ ] ✨ New feature (a non-breaking change that adds functionality)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Resolves #2728.

Unicode characters display wrong in chrome network panel because the Content-Type of headers is text/plain without setting charset in indexed mode.

According to 7.1 The Text Content-Type, default character set is US-ASCII if charset is absent which caused the problem.

Screenshots:

📝 Checklist

• [x] I have linked an issue or discussion.
• [x] I have updated the documentation accordingly.