correct the validator

lib/topic_query.rb

@@ -17,14 +17,20 @@ class TopicQuery
         Integer === x && x >= 0
       end
 
-      array_int = lambda do |x|
-        Array === x && x.length > 0 && x.all? { |i| Integer === i || i.match?(/^-?[0-9]+$/) }
+      int = lambda do |x|
+        Integer === x || (String === x && x.match?(/^-?[0-9]+$/))
+      end
+
+      array_int_or_int = lambda do |x|
+        int.call(x) || (
+          Array === x && x.length > 0 && x.all?(&int)
+        )
       end
 
       {
         max_posts: zero_or_more,
         min_posts: zero_or_more,
-        exclude_category_ids: array_int
+        exclude_category_ids: array_int_or_int
       }
     end
   end

spec/requests/list_controller_spec.rb

@@ -33,6 +33,9 @@ RSpec.describe ListController do
     it "returns 200 for legit requests" do
       get "/latest.json?exclude_category_ids%5B%5D=69&exclude_category_ids%5B%5D=70&no_definitions=true&no_subcategories=false&page=1&_=1534296100767"
       expect(response.status).to eq(200)
+
+      get "/latest.json?exclude_category_ids=-1"
+      expect(response.status).to eq(200)
     end
 
     it "doesn't throw an error with page params as an array" do