fix serialization of primitives in Response

[devkral]

Checklist

• The code has 100% test coverage.
• The documentation was properly created or updated (if applicable) following the correct guidelines and appropriate language.
• I branched out from the latest main or is a sub-branch.

Summary or description

Before:

When passing e.g. an int or float as a content in make_response and using a normal Response a crash was the result.

After:

Handle everything gracefully.

This bug is old but was only exposed by a new test.