[3.2 -> main] Return application/json for all http responses including errors

[heifner]

Revert to 3.1 behavior of always returning application/json for all http responses including errors. The non-JSON responses came in as part of the boost beast backport #22.

Example:

curl -v http://localhost:8888/hi
*   Trying 127.0.0.1:8888...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 8888 (#0)
> GET /hi HTTP/1.1
> Host: localhost:8888
> User-Agent: curl/7.68.0
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 404 Not Found
< Content-Type: application/json
< Server: Boost.Beast/266
< Access-Control-Allow-Origin: *
< Content-Length: 212
<
* Connection #0 to host localhost left intact
{"code":404,"message":"Not Found","error":{"code":0,"name":"exception","what":"unspecified","details":[{"message":"Unknown Endpoint","file":"beast_http_session.hpp","line_number":159,"method":"handle_request"}]}}%

Resolves #324
Merges #326 into main