Mirrored request with host rewrite has wrong Host header

*Description*:
I'm currently trying to use `request_mirror_policy` in a route that is also using `auto_host_rewrite`.
The problem are the `host/:authority` headers in the mirrored request is giving me weird results...

*Repro steps*:
I created a simplified version of my setup in this [gist](https://gist.github.com/Brunomachadob/33c53e0c3557283eb2e802cc2699fc39) including a full log of execution

```bash
docker-compose up --build
```
```bash
curl localhost:5000/any/host_rewrite
```

Mock server logs 2 requests
```bash
# The actual request
mockserver_1 | {
mockserver_1 | "method" : "GET",
mockserver_1 | "path" : "/any/host_rewrite",
mockserver_1 | "headers" : [ {
mockserver_1 | "name" : "host",
mockserver_1 | "values" : [ "api" ] # successfully rewritten
mockserver_1 | }, {
....
```
```bash
# The mirror request
mockserver_1 | {
mockserver_1 | "method" : "GET",
mockserver_1 | "path" : "/any/host_rewrite",
mockserver_1 | "headers" : [ {
mockserver_1 | "name" : "host",
mockserver_1 | "values" : [ "localhost-shadow:5000" ] # Why localhost?
mockserver_1 | }, {
....
```

Calling again...

```bash
curl localhost:5000/any/host_rewrite
```

Mock server logs 2 requests
```bash
# The actual request
mockserver_1 | {
mockserver_1 | "method" : "GET",
mockserver_1 | "path" : "/any/host_rewrite",
mockserver_1 | "headers" : [ {
mockserver_1 | "name" : "host",
mockserver_1 | "values" : [ "api" ] # successfully rewritten
mockserver_1 | }, {
```
```bash
# The mirrored request
mockserver_1 | {
mockserver_1 | "method" : "GET",
mockserver_1 | "path" : "/any/host_rewrite",
mockserver_1 | "headers" : [ {
mockserver_1 | "name" : "host",
mockserver_1 | "values" : [ "api-shadow" ] # successfully rewritten
```

From now on it's always successfully rewritten, until I restart the containers and the same thing happen. Maybe this is some caching problem in the host resolution?

*Tested versions*
`envoyproxy/envoy:v1.10.0`
`envoyproxy/envoy-dev:daeb9850dbcd78da21a2e8ff9854564830cefb19 # 1.13.0-dev`

*Additional question*
As you can see in my `envoy.yaml` I want to mirror the request to another cluster with another domain (I'm migrating one feature from a monolith to a microservice).

So in my case the real request is going to `api`, and the mirror request is going to another cluster called `api-mirror`. In this case, shouldn't the request mirror get the `host=api-mirror-shadow`?

The [documentation](https://www.envoyproxy.io/docs/envoy/latest/api-v2/api/v2/route/route.proto#envoy-api-msg-route-routeaction-requestmirrorpolicy) is not really clear (at least to me) which cluster is going to be used when rewriting, the mirror cluster or the original one?

Thanks for the help!