asset guide, match application.rb snippets to the generated file [ci skip]

`if defined?(Bundler)` is no longer used in our current application.rb
cleanup, switch arguments in assert_equal calls
cleanup, assert on warning from postgres adapter
Fix that #pluck wasn't rescuing ThrowResult, meaning it would blow up when failing to construct_limited_ids_condition.
Add coverage for the fact that pluck without an argument returns all the table's columns.
No point in memoizing a simple literal string.
Make NullRelation a bit more like a real relation by returning 0 for #calculate(:count)
Rather than raising ThrowResult when construct_limited_ids_conditions comes up empty, set the relation to NullRelation and rely on its results.

This will help avoid errors like 2fcafee250ee2, because in most cases NullRelation will do the right thing. Minor bonus is avoiding the use of exceptions for flow control.
Document ActionMailer::Base#set_content_type
Adding notes on PATCH to the upgrade guide.

Discusses compatibility between PATCH and PUT, as well as how to add
support for JSON PATCH to your application.

Fixes #10439.
emphasize that callbacks are called in destroy_all

Cleaned up rdoc a bit emphasizing that callbacks are called. Also
removed the stress on the fact that records are always removed.

If callbacks return false then records will not be deleted.
fix grammar
Added documentation for Rails::Generators::NamedBase.template
Added documentation for model migration generation
improved doc for ActiveRecord#find_by_sql method (Refs #10599) [ci skip]
Use new hash syntax
ActionMailer::Collector should be nodoc'd
Document ActionMailer::LogSubscriber and ActionMailer::DeliveryMethods::ClassMethods
Document ActionMailer::MailHelper
nodoc ActionMailer module from lib/action_mailer/*.rb