def authors(commit_id = nil)
      authors = ActorCollection.new
      if commit_id.nil?
        each { |commit| authors << commit.author }
      elsif key? commit_id
        authors << self[commit_id].author
      end
      authors
    end
def authors(commit_id = nil)
      authors = ActorCollection.new
      if commit_id.nil?
        each { |commit| authors << commit.author }
      elsif key? commit_id
        authors << self[commit_id].author
      end
      authors
    end
Returns the authors of all or a specific commit in this collection

 @param [Object] commit_id The ID of the commit, if only the author of a
        specific commit should be returned
 @return [ActorCollection] All authors of the commits in this collection
         or the author of a specific commit
 @see Commit#author