How to fix ActionController::UnknownFormat error for json rendered reponse in RSpec

Question

Some of the specs are failing While upgrading ruby, rails versions.

More often for the controller actions which are rendering JSON and XML responses.

Old versions: ruby -v (1.9.3p551), rails -v (3.0.13), rspec -v (2.4.0)

New Versions: ruby -v (2.4.4p296), rails -v (4.2.11), rspec -v (2.99.2)

I have tried by mentioning format as well.

describe "GET index" do
    it "assigns all users as @users" do
        get :index, params, format: :json
    end
end

app/controllers/users_controller.rb

class UsersController < ApplicationController
respond_to :json,:xml, :html

def index
    @users = User.where(:gciunt => params[:gciunt])
        respond_to do |format|
            format.json {render :json => @users, :callback => params[:callback]}
        end
    end
end

spec/controllers/users_controller_spec.rb

require 'spec_helper'

describe UsersController do
    def mock_user(stubs={})
        (@mock_user ||= mock_model(User).as_null_object).tap do |user|
            user.stub(stubs) unless stubs.empty?
        end
    end

    describe "GET index" do
        it "assigns all users as @users" do
            params = {:gciunt => "37"}
            User.should_receive(:where).with(params) { [mock_user] }
            get :index, params, format: :json
            assigns(:users).should eq([mock_user])
        end
    end
end

Error
<b>Failure/Error: get :index, params
ActionController::UnknownFormat:
ActionController::UnknownFormat
# ./app/controllers/users_controller.rb:6:in `index'
# ./spec/controllers/users_controller_spec.rb:15:in `block (3 levels) in <top (required)>'<b>

This error is occurring only after upgrading ruby, rails, RSpec.

Please find the Attached Image here