The current header parsing logic splits headers on all colons, which breaks headers containing colons in their values, such as Content-Security-Policy with URLs (https://example.com:443) or other complex directives.

This PR changes the header parsing to only split on the first colon, preserving any subsequent colons in the header value. This allows proper handling of:

• Content-Security-Policy with URLs and complex directives

• Custom headers with colons in values

• Other standard headers that may contain colons

• Before the fix:

HEADER=Content-Security-Policy:default-src 'self'; style-src 'self' https://example.com:443

Would incorrectly split into multiple parts at each colon.

• After the fix:

The header value is properly preserved with all its colons intact.

Added tests to verify proper handling of complex header values.