Added method `#eql?` to `ActiveSupport::Duration`, in addition to `#==`.

Currently, the following returns `false`, contrary to expectation:

    1.minute.eql?(1.minute)

Adding method `#eql?` will make this behave like expected. Method `#eql?` is
just a bit stricter than `#==`, as it checks whether the argument is also a
uration. Their parts may be different though.

    1.minute.eql?(60.seconds)  # => true
    1.minute.eql?(60)          # => false
Update RecordNotFound exception cases to include a message with the
Model that the Record was not found in.
'Reversible' ->active_record_migrations [ci skip]

Reversible method documentation moved from migrations.md  to active_record_migrations.md
'Revert' ->active_record_migrations [ci skip]

Revert method documentation moved from migrations.md  to active_record_migrations.md
[ci skip] Fix some typos, normalize sentence.

Follow up of #16722.

https://github.com/rails/rails/pull/16722

also normalize all instance gained methods’ sentence.
Explain ERB space removal. [ci skip]
Changes default render behavior from file to template.
form_helpers: demonstrating there is also a collection_select method in form builder
Use dynamic path for dummy app location in plugin's test_helper.rb
Don't autosave unchanged has_one through records
[ci skip] Fix code snippet display in Action Mailer Basics guide.
Make explicit 4.1 is being maintained by a special situation

[ci skip]
Merge pull request #16914 from untidy-hair/fix_plugin_dummy_path_error

Use dynamic path for dummy app location in plugin's test_helper.rb
Merge pull request #16790 from cirosantilli/explain-erb-space-removal

Explain ERB space removal.

[ci skip]
Added Responders docs, made Web Console docs more consistent. [ci skip]
Backwards compat for activerecord-deprecated_finders

Maintain compatibility for:
* ActiveRecord::Base#find(:all)
* ActiveRecord::Base#find(:first)
* ActiveRecord::Base#find(:last)
[guides] 4.2 release notes: added technical details for Adequate Record
Improve environment config templates consistency:

* Fix long lines;
* Use simple quotes for string literals when interpolation is not used.
Merge pull request #16841 from tjouan/environment-config-consistency

Improve environment config templates consistency
Merge pull request #16844 from zerothabhishek/master

[guides] 4.2 release notes: added technical details for Adequate Record