Trust ping is always returns response synchronously regardless of return_route header value

[antonbaliasnikov]

Bug overview

Accordingly to the return_route extension, if the header specified with value none or omitted, then the trust ping protocol response should be asynchronous to the service_endpoint specified in the recipient DID document.

RootsID mediator ignores the return_route and always responds in the same connection synchronously to trust the ping message.

Expected behavior

1. If return_route equals to all, then sync response (as now supported) is returned
2. if return_route is none or omitted, then async response is expected (not supported)