find keys also in assigned collections

lib/jekyll/drops/site_drop.rb

@@ -19,6 +19,10 @@ module Jekyll
         end
       end
 
+      def key?(key)
+        (@obj.collections.key?(key) && key != "posts") || super
+      end
+
       def posts
         @site_posts ||= @obj.posts.docs.sort { |a, b| b <=> a }
       end

test/test_site_drop.rb

@@ -0,0 +1,21 @@
+require "helper"
+
+class TestSiteDrop < JekyllUnitTest
+  context "a site drop" do
+    setup do
+      @site = fixture_site({
+        "collections" => ["thanksgiving"]
+      })
+      @site.process
+      @drop = @site.to_liquid.site
+    end
+
+    should "respond to `key?`" do
+      assert @drop.respond_to?(:key?)
+    end
+
+    should "find a key if it's in the collection of the drop" do
+      assert @drop.key?("thanksgiving")
+    end
+  end
+end