Cleaning up the changes made by @vapir.

lib/google/api_client/discovery/schema.rb

@@ -47,8 +47,12 @@ module Google
           # and excess object creation, but this hopefully shouldn't be an
           # issue since it should only be called only once per schema per
           # process.
-          if data.kind_of?(Hash) && data['$ref'].is_a?(String)
-            reference = data['$ref']
+          if data.kind_of?(Hash) && data['$ref']
+            if data['$ref'].respond_to?(:to_str)
+              reference = data['$ref'].to_str
+            else
+              raise TypeError, "Expected String, got #{data['$ref'].class}"
+            end
             reference = '#' + reference if reference[0..0] != '#'
             data.merge({
               '$ref' => reference