Require action_view to fix missing constant

Previously, requiring action_view/view_paths did cause an uninitialized
constant error for ENCODING_FLAG, which is defined in action_view.
[Testing Guide] Mention :unauthorized in assert_response explanation [ci skip]
Merge pull request #13967 from fphilipe/fix-missing-action-view-const

Require action_view to fix missing constant ENCODING_FLAG
Merge pull request #13952 from prathamesh-sonpatki/testing

[Testing Guide] Mention :unauthorized in assert_response explanation [ci skip]