code: func (c *Call) Run(fn func(args Arguments)) *Call {
  	c.lock()
  	defer c.unlock()
  	c.RunFn = fn
  	return c
  }
docs: // Run sets a handler to be called before returning. It can be used when
  // mocking a method such as unmarshalers that takes a pointer to a struct and
  // sets properties in such struct
  //
  //    Mock.On("Unmarshal", AnythingOfType("*map[string]interface{}").Return().Run(func(args Arguments) {
  //    	arg := args.Get(0).(*map[string]interface{})
  //    	arg["foo"] = "bar"
  //    })