Prepare tests for supporting QUERY method.

Put message key in its correct location.

Include basic description of the error and the steps to resolve them.

Include example project and requests.

Tidy up some parts of the codebase.

Put data source to its own package.

Prepare project for multiple packages.

Include tests for emplace, modify fallback content negotiation logic.

Decouple tests from data source.

Decouple tests from data source.

Add tests related to error handling.