enhancement: add a single space after colon in headers in Response class

[rasteiner]

Description

This PR adds a single space after the : in header responses for better server support.

Summary of changes

Add a single space after colon in headers in Response class.

Before:

Content-Type:foo/bar

After:

Content-Type: foo/bar

Reasoning

That single space was expected in HTTP/1.0.
While since HTTP/1.1 it's optional, some products still seem to rely on it being present. So, adding it broadens the support for servers (and really old browsers I guess).

Additional context

HTTP/1.0: https://datatracker.ietf.org/doc/html/rfc1945#section-4.2
HTTP/1.1: https://datatracker.ietf.org/doc/html/rfc2616#section-4.2

Changelog

Fixes

• Panel API routes not working with FrankenPHP in Kirby 4 #6975

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add lab and/or sandbox examples (wherever helpful)
• Add changes & docs to release notes draft in Notion