Expand tilde paths

I'd like to specify "~/.env" as an env file but File.exists? returns false for that regardless of whether it actually exists. A simple call to File.expand_path would fix this.

I updated the shared example group and added a test. Unfortunately I had to mock File.exists? to make it believe that "~/.env" exists, but that can't be helped unless the specs are modified to use something like the fakefs gem.