Add test ensuring local gem downloads overwrite stale cache

[afurm]

What was the end-user or developer problem that led to this PR?

Gem::RemoteFetcher#download has a nil-scheme branch for local file paths, but there was no test proving that a local .gem source replaces a stale cached gem. The missing coverage left a regression risk and kept a TODO in place.

What is your fix for the problem, implemented in this PR?

Added a focused test that seeds the cache with stale contents, points source_uri to a local .gem, and asserts the cache is overwritten with the local file’s contents. The change exercises the nil-scheme code path and removes the obsolete TODO.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags