[ruby-core:97877] [Ruby master Bug#16785] ruby 2.8.0-dev 5c27681813 causes Rails CI failure

Issue #16785 has been reported by yahonda (Yasuo Honda).

----------------------------------------
Bug #16785: ruby 2.8.0-dev 5c27681813 causes Rails CI failure
https://bugs.ruby-lang.org/issues/16785

* Author: yahonda (Yasuo Honda)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-04-12T03:45:22Z master 5c27681813) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
Rails CI has been failing since https://buildkite.com/rails/rails/builds/68202#ee7fdd62-d66b-48a1-a55f-af28a2bff287 , according to git bisect it is triggered since ruby 2.8.0-dev 5c27681813

# Steps to reproduce (I have not found how to reproduce without Rails framework)

```
git clone https://github.com/rails/rails
cd rails/actionpack
bundle install
PARALLEL_WORKERS=1 bin/test test/controller/base_test.rb  --seed 31701 -n "/^(?:UrlOptionsTest#(?:test_url_options_override)|ControllerInstanceTests#(?:test_performed\\?)|PerformActionTest#(?:test_action_missing_should_work))$/"
```

# Expected result
It should pass as ruby 2.8.0-dev a01bda594996fdc247e183f107cada43e2c8e3b8 or prior versions.

# Actual result:

```
 % PARALLEL_WORKERS=1 bin/test test/controller/base_test.rb  --seed 31701 -n "/^(?:UrlOptionsTest#(?:test_url_options_override)|ControllerInstanceTests#(?:test_performed\\?)|PerformActionTest#(?:test_action_missing_should_work))$/"
Run options: --seed 31701 -n "/^(?:UrlOptionsTest#(?:test_url_options_override)|ControllerInstanceTests#(?:test_performed\\?)|PerformActionTest#(?:test_action_missing_should_work))$/"

# Running:

.E

Error:
PerformActionTest#test_action_missing_should_work:
ActionController::MissingExactTemplate: ActionMissingController#["Response for arbitrary_action"] is missing a template for request formats: text/html
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/metal/implicit_render.rb:45:in `default_render'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/metal/basic_implicit_render.rb:6:in `block in send_action'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/metal/basic_implicit_render.rb:6:in `tap'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/metal/basic_implicit_render.rb:6:in `send_action'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/abstract_controller/base.rb:195:in `process_action'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/metal/rendering.rb:30:in `process_action'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/abstract_controller/callbacks.rb:42:in `block in process_action'
    /Users/yahonda/src/github.com/rails/rails/activesupport/lib/active_support/callbacks.rb:98:in `run_callbacks'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/abstract_controller/callbacks.rb:41:in `process_action'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/metal/rescue.rb:22:in `process_action'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/metal/instrumentation.rb:34:in `block in process_action'
    /Users/yahonda/src/github.com/rails/rails/activesupport/lib/active_support/notifications.rb:203:in `block in instrument'
    /Users/yahonda/src/github.com/rails/rails/activesupport/lib/active_support/notifications/instrumenter.rb:24:in `instrument'
    /Users/yahonda/src/github.com/rails/rails/activesupport/lib/active_support/notifications.rb:203:in `instrument'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/metal/instrumentation.rb:33:in `process_action'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/metal/params_wrapper.rb:245:in `process_action'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/abstract_controller/base.rb:136:in `process'
    /Users/yahonda/src/github.com/rails/rails/actionview/lib/action_view/rendering.rb:39:in `process'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/metal.rb:190:in `dispatch'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/test_case.rb:517:in `process'
    /Users/yahonda/src/github.com/rails/rails/actionpack/lib/action_controller/test_case.rb:395:in `get'
    /Users/yahonda/src/github.com/rails/rails/actionpack/test/controller/base_test.rb:173:in `test_action_missing_should_work'


bin/test Users/yahonda/src/github.com/rails/rails/actionpack/test/controller/base_test.rb:171

..

Finished in 0.128453s, 23.3548 runs/s, 38.9247 assertions/s.
3 runs, 5 assertions, 0 failures, 1 errors, 0 skips
%
```





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