Support RecursiveField with 'many' option (#321)

tests/reference.yaml

@@ -1617,6 +1617,7 @@ definitions:
     required:
       - title
       - children
+      - many_children
     type: object
     properties:
       id:
@@ -1632,6 +1633,10 @@ definitions:
         type: array
         items:
           $ref: '#/definitions/TodoTree'
+      many_children:
+        type: array
+        items:
+          $ref: '#/definitions/TodoTree'
   TodoYetAnother:
     required:
       - title