add implementation of respond_to? for ActiveSupport::Duration
Merge pull request #16574 from lsylvester/duration-respond_to

add implementation of respond_to? for ActiveSupport::Duration
As of Unicode 6.3, Mongolian Vowel Separator is not whitespace

Ruby 2.2 knows this, and no longer matches it with [[:space:]], so it's
not a good candidate for testing String#squish.
Don't autosave unchanged has_one through records
[ci skip] Fix typo `equals to` --> `equal to`
Merge pull request #16927 from akshay-vishnoi/fix-typo

[ci skip] Fix typo `equals to` --> `equal to`
[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
Consistently capitalize ETag.
Consistently markup etag options.
Specify dummy app's db migrate path in plugin's test_helper.rb
pg, correctly dump foreign keys targeting tables in a different schema.

Closes #16907.

[Matthew Draper & Yves Senn]