Fix linter offenses not updating after re-opening a file.
Fix linter offenses not updating after re-opening a file.

Need to check whether the document provided to the linter is still the
same object as in previous runs. I.e. when closing and re-opening a file,
the document object changes, so the existing linter object won't see
updates to the content anymore.

rubyide/vscode-ruby,rubyide/vscode-ruby,rubyide/vscode-ruby,rubyide/vscode-ruby,rubyide/vscode-ruby