Stringify error extensions (fixes #2246)

Author: cmatheson

lib/graphql/execution_error.rb

@@ -48,8 +48,9 @@ def to_h
         hash.merge!(options)
       end
       if extensions
-        hash["extensions"] ||= {}
-        hash["extensions"].merge!(extensions)
+        hash["extensions"] = extensions.each_with_object({}) { |(key, value), ext|
+          ext[key.to_s] = value
+        }
       end
       hash
     end

spec/graphql/query/executor_spec.rb

@@ -232,7 +232,7 @@
                 "message"=>"Error was handled!",
                 "locations" => [{"line"=>1, "column"=>17}],
                 "path"=>["error"],
-                "extensions"=>{code: "DUMMY_ERROR"}
+                "extensions"=>{"code" => "DUMMY_ERROR"}
               }
             ]
           }

spec/support/dummy/schema.rb

@@ -419,7 +419,7 @@ def execution_error_with_options
 
     field :execution_error_with_extensions, Integer, null: true
     def execution_error_with_extensions
-      GraphQL::ExecutionError.new("Permission Denied!", extensions: { "code" => "permission_denied" })
+      GraphQL::ExecutionError.new("Permission Denied!", extensions: { code: "permission_denied" })
     end
 
     # To test possibly-null fields