It looks like what is happening when you set a default value for an int_list parameter in this way:
-- int_list :categories = "3, 5, 6"
is that the following gets run:
'"1'.downcase.to_i
That will return 0.
You can get around the issue by leaving the quotes off the default parameter:
-- int_list :categories = 3, 5, 6
That value still gets interpreted as a string before being split into an array. Maybe the plugin should strip outer quotation marks if they get added to a string input.