Fix JSON parse error when backend returns 304

[OttoWinter]

For some URLs (namely /ping and /serial-ports) tornado will automatically set an ETag Header with the hashed payload.

When there's a cache hit, the backend only returns 304 Not Modified.

HTTP Request/response:

GET /serial-ports HTTP/1.1
Host: localhost:6052
# ...
If-None-Match: "49b6fc370dd17e5696abd7a3294d834fac2cccd5"
Cache-Control: max-age=0


HTTP/1.1 304 Not Modified
Server: TornadoServer/6.1
Date: Thu, 05 Aug 2021 16:40:49 GMT
Etag: "49b6fc370dd17e5696abd7a3294d834fac2cccd5"

Importantly, for 304 Not Modified the content-type header is not sent (the HTTP RFC also specifies 304 to act just like that https://stackoverflow.com/a/31107285)

I know it's an ugly fix, but I couldn't find a way to disable the behavior in the backend (tornado has horrible docs). So instead fix it in the frontend (where ideally it should be fixed anyway, because the HTTP RFC recommends this behavior)