Feature/adding integration tests

[jamie-taylor-rjj]

Rationale for this PR

This PR simplifies a the tests uses for the default middleware configuration down to a single integration test. As there is now an integration test for the default set up, a lot of the unit tests which were testing the default set up have been removed.

The code base now uses file-scoped namespaces.

PR Checklist

Feel free to either check the following items (by place an x inside of the square brackets) or by replacing the square brackets with a relevant emoji from the following list:

• ✅ to indicate that you have checked something off
• ❎ to indicate that you haven't checked something off
• ❓ to indicate that something might not be relevant (writing tests for documentation changes, for instance)

Essential

These items are essential and must be completed for each commit. If they are not completed, the PR may not be accepted.

• [❎] I have added tests to the OwaspHeaders.Core.Tests project
• [✅] I have run the dotnet-format command and fixed any .editorconfig issues
• [✅] I have ensured that the code coverage has not dropped below 65%
• [✅] I have increased the version number in OwaspHeaders.Core.csproj (only relevant for code changes)

Optional

• [❓] I have documented the new feature in the docs directory
• [❎] I have provided a code sample, showing how someone could use the new code

Any Other Information

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