I just hit the URL above in Google Chrome and did not get the upgrade to https.
Here are the request headers from developer mode.

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Connection: keep-alive
DNT: 1
Host: assets.fanart.tv
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/98.0.4758.102 Safari/537.36

Before going through many hurdles, you might consider trying to upgrade your ProntoScript to send this header but with a value of 0.

Also, if you want to try things out without using curl, I highly recommend PostMan as a test platform as you can 'tweak' the headers.