fix: Build the URL using url.JoinPath instead of path.Join

[MikeGoldsmith]

Which problem is this PR solving?

There is a weird bug in setting a URL path directly, where it will escape some characters, but not all.

This go playground example shows it. The unescaped value is used directly, the escaped value double escapes some of the characters.

This PR uses url.PathJoin instead, which is intended to handle URLs.

Short description of the changes

• Update the URL builder to use url.JoinPath and return any errors
• Update Transmission to use updated builder func and handle error like before
• Update tests to handle errors

[kentquirk]

Looks good!