`Document#fetch!` should correctly deal with unexpected redirects

[hmans]

• [ ] When encountering a 301 redirect, the Fetch service class should correctly return the requested document's new URL in data["url"].
• [ ] If this URL differs from the one originally passed to Fetch, check if there is another Pants::Document already available for the new URL.
  • [ ] If there is, update that document instead and discard (destroy) the other instance.
  • [ ] If there isn't, continue as before and update the document instance with the new URL.
• [ ] Q: what if the new URL also has a different host? The user may have moved to a new host. How do we deal with this?

[hmans]

Updated the issue with a new implementation strategy.