Adding a mapping with a description to `WireMockServer.WithMapping`
did not include the description to the resulting
`IRespondWithAProvider`, which means that calling
`WireMockServer.SavePact` does not populate the description in the
contract file.
This PR includes the description when mapping from `MappingModel` to
`IRespondWithAProvider`.
Since the root certificate of client_cert.pfx is obviously not trusted automatically by cloning this repo, the tests in WireMockServerTests.ClientCertificate.cs set WireMockServerSettings.AcceptAnyClientCertificate to true so that tests pass even if the device hasn't trusted the root of client_cert.pfx.