fix(schema): throw error if versionKey is not a string

Author: vkarpov15

lib/schema.js

@@ -412,6 +412,10 @@ Schema.prototype.defaultOptions = function(options) {
     options.read = readPref(options.read);
   }
 
+  if (options.versionKey && typeof options.versionKey !== 'string') {
+    throw new MongooseError('`versionKey` must be falsy or string, got `' + (typeof options.versionKey) + '`');
+  }
+
   if (options.optimisticConcurrency && !options.versionKey) {
     throw new MongooseError('Must set `versionKey` if using `optimisticConcurrency`');
   }