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
Remove unnecessary require from active_support/inflector/methods.rb

`active_support/inflections` already requires
`active_support/inflector/inflections`. There's no need to require it in
`active_support/inflector/methods`.
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.
change to destructive `deep_symbolize_keys` after https://github.com/rails/rails/commit/df24b8790f22384a068fece7042f04ffd2fcb33e which allows to do so. This helps to avoid extra hash object creation, by symbolizing inplace
Fixes bug 10628.
make "rails dbconsole" work with activerecord-postgis-adapter
Eliminate minitest warnings

https://github.com/seattlerb/minitest/commit/9a57c520ceac76abfe6105866f8548a94eb357b6#L15R8
Update filter_parameters related docs, as they have been moved to initializers.
Update configuration docs for assets.enabled, as it's set true by default in the configuration and doesn't appear in the config files anymore.
update_counters accepts a hash, not an array of hashes
Documentation: Notifications queue does not run in a thread.
Extract AR::Core#inspect and AR::Core.inspect's test code from base_test.rb

The methods got moved to core.rb in commit
b2c9ce341a1c907041f55461aefebb0321280cb5, but the tests never did.
Extract AR::AttributeMethods#attribute_for_inspect's test code out from base_test.rb

The method itself got extracted out from ActiveRecored::Base in commit
ceb33f84933639d3b61aac62e5e71fd087ab65ed, but the test code never did.
use drop rather than calculate the array length