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.
Fixes bug 10628.
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.
pass where values to the helper function rather than rely on internal state
save the where values in variables so we don't need to look them up all
the time
change method name to reflect what it actually does.
eliminate some conditionals
partition the where values so we can access the removed ones
update_counters accepts a hash, not an array of hashes
Documentation: Notifications queue does not run in a thread.
In batches test @total was assigned but not used. Use it in tests instead of Post.count