Trust-ping messages of string type are ignored

[conanoc]

Steps to reproduce:

1. Aries agent Alice starts the mediation protocol with a mediator.
2. After getting a grant message, Alice sends a trust-ping message with return-route: all as a string type websocket message.
3. Alice gets no response from the mediator.
4. Alice sends a trust-ping message with return-route: all as binary type.
5. Alice gets a ping-response message normally.

Tested on https://indicio-tech.github.io/mediator/
This seems to be related to Caddy reverse-proxying because I cannot reproduce this issue when I test it using ACA-Py 0.7.3 without Caddy.

WebSocket messages can be sent as either string type or binary type. So, this issue is not a big deal, but it took me much time in debugging.