Fix the generated code for enum-based map keys for json services (#1602)

* Fix the generated code for enum-based map keys for json services

* Fix the changelog test for changes in the CodeGenerator

The CodeGenerator package does not have a changelog as it has no
releases.

src/Generator/RequestSerializer/RestJsonSerializer.php

@@ -227,8 +227,8 @@ private function dumpArrayMap(string $output, string $input, string $contextProp
         $mapKeyShape = $shape->getKey()->getShape();
         if (!empty($mapKeyShape->getEnum())) {
             $enumClassName = $this->namespaceRegistry->getEnum($mapKeyShape);
-            $validateEnum = strtr('if (!ENUM_CLASS::exists($mapKey)) {
-                    throw new InvalidArgument(sprintf(\'Invalid key for "%s". The value "%s" is not a valid "ENUM_CLASS".\', __CLASS__, $mapKey));
+            $validateEnum = strtr('if (!ENUM_CLASS::exists($name)) {
+                    throw new InvalidArgument(sprintf(\'Invalid key for "%s". The value "%s" is not a valid "ENUM_CLASS".\', __CLASS__, $name));
                 }', [
                 'ENUM_CLASS' => $enumClassName->getName(),
             ]);