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]
Introduce `render :body` as an alias for `:text`
Update hash format for render_text_test
Add tests to confirms render response format

Both `render :text` and `render :body` should render with a default
content type.
Introduce `render :plain`

`render :plain` renders a given content in `text/plain` format, unlike
`:body` and `:text` which renders the content and inherits the default
`text/html` content type.