Do not autoupdate unless URL is accessible

[Ash258]

Implemented in Shovel

As soon as manifest specify hash extraction, and hash is found, URL is not checked. This lead to possible problem when hash was extracted correctly, but installation failed since developers changed URL.

• Ash258/Shovel-Ash258@3ad6cd5 -> Ash258/Shovel-Ash258@02af274
• ScoopInstaller/Extras@f6a7c70 -> ScoopInstaller/Extras@8b3c4dd

Change should be simple:

https://github.com/lukesampson/scoop/blob/a9fa775d59b14e7dce335313faa0eff855469764/lib/autoupdate.ps1#L262

This line need to test HTTP Status and then return hash. Cannot think right now how to test remote file existence and do not download full file. It should be possible to download few bytes and see status