add in forgotten context mutation (#68)

Author: jakesower

lib/index.js

@@ -609,13 +609,18 @@ module.exports = Serializer => {
       const isArray = recordTypes[originalType][relatedField][keys.isArray]
       const isNull = !isArray && payload[reservedKeys.primary] === null
 
-      if (isNull)
+      if (isNull) {
+        // Rewrite type and IDs.
+        contextRequest.type = originalType
+        contextRequest.ids = null
+
         return [{
           id: originalIds[0],
           replace: {
             [relatedField]: null
           }
         }]
+      }
 
       if (originalIds.length > 1)
         throw new NotFoundError(