Allow to set custom content type for email attachments

With Action Mailer it's now easy to specify the content type of your attachments. E.g.: mail(body: "<h1>Hello</h1>", content_type: "text/html").

Added option to ActiveRecord::CounterCache methods

You can now ask Active Record to update specific timestamp columns when incrementing, decrementing, resetting, or updating counter caches.

Default Rails.env to development when missing

If your app does not set RAILS_ENV or RACK_ENV then the value of Rails.env will fall back to development rather than just being an empty string.

Improve Array#sum with Ruby refinements

Array#sum was defined in Rails with a monkey-patch. Ruby 2.0 introduced refinements for better encapsulation. This is the first PR where they are being used in rails/rails.

Ensure Rails 4.2 works with Ruby 2.4

Some tests and code have been fixed and backported to 4-2-stable so that running Rails 4.2 on Ruby 2.4 will not raise any error. 

Change return value of `duplicable?`

In Ruby 2.4, NilClass, FalseClass, TrueClass, Symbol and Numeric all support dup. For consistency, duplicable? will now return true for all these classes.

Serialize JSON attribute value nil as SQL NULL

The way in which a nil JSON payload is stored in a database was inadvertently changed in Rails 5.0. This fix maps nil to SQL NULL, exactly as how it was in Rails 4.x.

Fix generator command for nested namespaced rails engines

If the name of your Rails engine has a hyphen then this PR (fixing the path of namespaced controllers) is for you! 

Update `cookies` helper on all HTTP requests

This fixes a regression by which cookies were only updated on GET requests. Now we will update the helper for all requests.

Remove support for MySQL

MySQL 5.0 reached "end of life" more than five years ago. Rails now requires at least version 5.1.10.

Remove previously deprecated code

By now, you should have already dropped any reference to Rake's db:test:clone, Configuration's serve_static_files and static_cache_control and ActiveRecord's uniq, uniq!, uniq_value, insert_sql, update_sql, delete_sql.