[ci skip] Add explanation on Rack::Sendfile and fix serve static files setting.
store join parts in a tree
generate the association hash from the top down

This prevents us from walking back up the parent tree once for each
leaf.
do not convert the tree to a list just for the first node
walk the tree rather than generating a hash and walking it
rename the variable to make more sense with a tree
Add documentation for after_touch [ci skip]
reduce number of comparisons and array allocations

Just inspecting the children nodes will reduce the number of comparisons
we have to do (since it is limited to the child size)
parent is guaranteed to be the same

since we are searching a tree, the current level should be consistent,
so we don't have to inspect the parent class / table
we do not need to to_s the name all the time
we will always detect the same node, so just pass the node in
eliminate function that is only used in one place
names are guaranteed to be symbols
Define the association extensions without need to have a builder
instance
Merge pull request #12473 from claudiob/document-after-touch

Add documentation for after_touch [ci skip]
Fixing repond_with working directly on the options hash

This fixes an issue where the respond_with worked directly with the given
options hash, so that if a user relied on it after calling respond_with,
the hash wouldn't be the same.

Fixes #12029
Merge pull request #12083 from BlueHotDog/fixing_respond_with

Fixing repond_with working directly on the options hash
add dots after `Fixes #YYYYY` in actionpack CHANGELOG. [ci skip]
Merge pull request #12479 from benpickles/unshift-middleware

Expose MiddlewareStack#unshift to environment configuration
Merge pull request #12283 from JuanitoFatas/rails-on-rack

[ci skip] Add explanation on Rack::Sendfile middleware in rails_on_rack.md.