default envoy config fails silenlty with custom metadata

Resolving this issue just cost me a few hours: with the [example envoy config](https://github.com/grpc/grpc-web/blob/master/net/grpc/gateway/examples/echo/envoy.yaml), all metadata keys except `custom-header-1` cause envoy to drop the request. This is very hard to track down, especially because envoy, even with debug output enabled, does not make clear that the disallowed header is the problem.

In my case, I am using a custom authorization scheme that employs the recommended `authorization` header. Perhaps it would make sense to include at least that theader in the sample config?