--dry-run (or -n) doesn't seem to trigger `when_writing`

[jasonkarns]

Given the following rakefile

  task "git_diff_rails_credentials:fix" do
    when_writing "set local git-config diff.rails_credentials" do
      sh "git config diff.rails_credentials.textconv 'bin/rails credentials:diff'"
    end
  end

When I run rake -n git_diff_rails_credentials:fix

I expect to see DRYRUN: set local git-config diff.rails_credentials in the output. But instead I see:

$ rake -n doctor:git_diff_rails_credentials:fix
** Invoke doctor:git_diff_rails_credentials:fix (first_time)
** Execute (dry run) doctor:git_diff_rails_credentials:fix

I assume this is related to #361 but there isn't any resolution there to indicate what the expectations are.