error default value of header append for ext_proc/ext_authz

The default value of `append` of `HeaderValueOption` should be true according to the API.

```
// Should the value be appended? If true (default), the value is appended to
// existing values. Otherwise it replaces any existing values.
// This field is deprecated and please use
// :ref:`append_action ` as replacement.
google.protobuf.BoolValue append = 2;
```

However, both the ext_authz & ext_proc use false as the default value of `append`.

Snippet of ext_proc:
```
const bool append = PROTOBUF_GET_WRAPPED_OR_DEFAULT(sh, append, false);
```

Snippet of ext_authz:
```
if (header.append().value()) {
authz_response->response_headers_to_add.emplace_back(
Http::LowerCaseString(header.header().key()), header.header().value());
} else {
authz_response->response_headers_to_set.emplace_back(
Http::LowerCaseString(header.header().key()), header.header().value());
}
```