Closes #398 - Fixed newsletter analysis to allow guard clause

[octowombat]

Following discussions with @jiegillet and an upstream code fix.

[jiegillet]

Sorry one last thing: for completeness, we should add tests for all 8 cases, something like

  test_exercise_analysis "open_log accepts 8 several variants",
    comments_exclude: [ElixirAnalyzer.Constants.newsletter_open_log_uses_option_write()] do
   [ defmodule Newsletter do
        import File
        def open_log(path) when is_binary(path) do
          open!(path, [:write])
        end
    end
  end,
   ....
]

[octowombat]

My pleasure