gateway: _redirects not being applied with If-None-Match headers

When loading websites via boxo/gateway implementation that use the _redirects feature (e.g. single page applications) browsers will sometimes give a 404 for the page instead of appropriately following the _redirects directive.

This happens due to browsers using the `If-None-Match` header to optimize page loads if the data may already be cached locally. However, due to a regression when doing a query with `If-None-Match` the _redirects directives are ignored and as a result the user sees a 404.

This should be fixed by #412 however if people are stuck on this problem in the meanwhile it can be circumvented by putting a proxy in front of the gateway that drops the `If-None-Match` header.