Insert middleware before Rack::ETag

> The length-hiding middleware adds random text (in the form of an HTML comment) to every page you serve. This can break HTTP caching / ETags for public pages since they are no longer identical on each request.

This can easily be resolved by letting `BreachMitigation::LengthHiding` do it's work after `Rack::ETag`.

I didn't test his, but seems fairly obvious.