Don't throw exception on already initialized

[ffMathy]

This makes it very painful to write integration tests using the preferred method (TestServer) in ASP .NET Core.

Because the statics are not cleaned up, we can't prevent the issue either in a pretty manner.

Besides, if you are configuring your stuff twice, you would already be having bigger issues.

[khellang]

There's now a forked version with some modernization and bugfixes now on NuGet.org. Hopefully this project will come back to life at some point and the fixes can be accepted upstream 😄