Use scheme instead of hardcoded port when testing URLs

What does this merge request do and why?

Uses the scheme to determine whether to use SSL when testing a URL. Currently SSL is used only if the port is 443, which does not work when using both HTTPS and a non-standard port.

Before:

└─[$]➜ gdk start --show-progress
...

=> Waiting until https://gdk.test:3443/users/sign_in is ready...................................... <continues until timeout is reached>

After:

└─[$]➜ gdk start --show-progress
...

=> Waiting until https://gdk.test:3443/users/sign_in is ready..
=> https://gdk.test:3443/users/sign_in is up (200 OK). Took 0.23 second(s).

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.