Fix --separate-models commandline option

fixes #585

lib/brakeman/options.rb

@@ -204,8 +204,8 @@ module Brakeman::Options
           options[:output_files].push(file)
         end
 
-        opts.on "--[no]-separate-models", "Warn on each model without attr_accessible (Default)" do |combine|
-          options[:collapse_mass_assignment] = combine
+        opts.on "--[no-]separate-models", "Warn on each model without attr_accessible (Default)" do |separate|
+          options[:collapse_mass_assignment] = !separate
         end
 
         opts.on "--summary", "Only output summary of warnings" do