Handle :log_if in Proxy constructors

[solnic]

This should be consolidated eventually, unfortunately the stdlib constructor got in the way 🙂 Anyhow, this fixes a bug where passing log_if as a kwarg option to add_backend with an external logger that should be proxied, would end up not setting this option on the proxy itself.

So, now this works:

Dry.Logger(:test).add_backend(Rollbar, log_if: :exception?)