Use File.join rather than URI.join

Using File.join is safer than URI.join, so the relative path does not get overridden.

There was a problem when an application had a relative path.

Say, for instance, an application was running on https://test.app/relative. When Chemlab attempted to navigate to a page, it was take the /relative path portion out because of the way URI.join behaves.

PoC:

$ irb
> URI.join('https://test.app/relative', '/path').to_s #=> https://test.app/path
> File.join('https://test.app/relative', '/path').to_s #=> https://test.app/relative/path