Add an alias to Header.all method

Hello there! It would be more convenient to add an alias to method Header.all to be Header.list_headers, or Header.list

Also, the comment/document would be more descriptive/obvious if it was

   # List all available headers
    # Get known header classes
    # @return [Array<Class>]
    def self.all
      return @header_classes if @header_classes
      @header_classes = @added_header_classes.values
    end

lemontree55 / packetgen

Add an alias to Header.all method #87