Details: HTTP header names should be case-insensitive per RFC 7230. When the users
configure the headers like 'user-agent' (lowercase), gatus was adding its own 'User-Agent'
header because the case-sensitive map lookup failed to find the existing header.
Fixed headers:
- User-Agent
- Content-Type
- Host
Fixes#1491"
Co-authored-by: TwiN <twin@linux.com>