Hop-by-hop abuse to malform header mutator

Summary

### Impact

Downstream services relying on the presence of headers set by the `header` mutator could be exploited. A client can drop the header set by the `header` mutator by including that header's name in the `Connection` header. Example minimal config:

```yaml - id: 'example' upstream: url: 'https://example.com' match: url: 'http://127.0.0.1:4455/' methods: - GET authenticators: - handler: anonymous authorizer: handler: allow mutators: - handler: header config: headers: X-Subject: {{ .Subject }} ```

``` curl -H "Connection: close,x-subject" http://127.0.0.1:4455/ ```

The `X-Subject` header will not arrive at the downstream server. It is completely dropped. In case the downstream server handles such a request in an unexpected way, an attacker can exploit this, assuming they know or guess the internal header name.

### Patches

c5cc7f736dc84185034be4356057d1c7a656d797

### Workarounds

The downstream server should handle the case that an expected header is not set by responding with an appropriate error.

### References

See background info in https://github.com/golang/go/issues/50580