Add test for unprocessable entity

Author: mhartl

test/integration/users_login_test.rb

@@ -12,6 +12,7 @@ def setup
     post login_path, params: { session: { email:    @user.email,
                                           password: "invalid" } }
     assert_not is_logged_in?
+    assert_response :unprocessable_entity
     assert_template 'sessions/new'
     assert_not flash.empty?
     get root_path
@@ -53,4 +54,4 @@ def setup
     log_in_as(@user, remember_me: '0')
     assert cookies[:remember_token].blank?
   end
-end
+end