MONGOID-5737 Fix issue where <=> errors if there's a non-document (#5910

)

* bug fix and test update

* typo

lib/mongoid/equality.rb

@@ -18,6 +18,7 @@ module Equality
     #
     # @return [ Integer ] -1, 0, 1.
     def <=>(other)
+      return super unless other.is_a?(Mongoid::Equality)
       attributes["_id"].to_s <=> other.attributes["_id"].to_s
     end
 

spec/mongoid/equality_spec.rb

@@ -189,6 +189,12 @@
     it "compares based on the document id" do
       expect(first <=> second).to eq(-1)
     end
+
+    it "doesn't break if one isn't a document" do
+      expect do
+        first <=> "Foo"
+      end.to_not raise_error
+    end
   end
 
   describe "#eql?" do