Force string for header value for default policy

[danielwestendorf]

Types of Changes

Fixes #81 by casting any non-nil && non-string value to a string. Since application code may set any header value to any object, forcing a string here makes sense.

• Bug fix.

Contribution

• [x] I added tests for my changes.
• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.