Via fails to proxy https://nodejs.dev/learn/introduction-to-nodejs in Firefox and Safari

Reported by user: https://app.hubspot.com/contacts/6291320/ticket/314041139/

Attempting to load https://via.hypothes.is/https://nodejs.dev/learn/introduction-to-nodejs in Firefox results in the error "Secure Connection Failed An error occurred during a connection to via.hypothes.is."

Using Safari results in "Safari Can’t Open the Page
Safari can’t open the page. The error is: “The operation couldn’t be completed. (kCFErrorDomainCFNetwork error 303.)” (kCFErrorDomainCFNetwork:303)"

Page loads fine in Chrome.

Note that adding a trailing slash "/" to the end of the URL allows FF and Safari to load the page successfully: https://via.hypothes.is/https://nodejs.dev/learn/introduction-to-nodejs/ . However, the final redirected address in the address bar has the trailing slash removed.

**To reproduce:**

1. In Firefox or Safari, attempt to load https://via.hypothes.is/https://nodejs.dev/learn/introduction-to-nodejs

**MacOS 11.2.1
Safari 14
Fireox 86**