Use inversed parent for first and last child of has_many association

Added X-Content-Type-Options to the header defaults.

With a value of "nosniff", this prevents Internet Explorer from MIME-sniffing a response away from the declared content-type.

Refactor multiparameter attributes assignment implementation.

Cleans up a lot of noise from arguments being passed from one method to
another.

This should make it easier to refactor and improve this code, and remove
complexity with params going around here and there.

Avoid doing a new column lookup for the attribute, since we already have
the column to check for the klass.

Move some methods to the top to better organize them, since they're used
right at the beginning of the multiparameter assignment method chain.

Replace deprecated `memcache-client` gem with `dalli` in ActiveSupport::Cache::MemCacheStore

memcache-client was deprecated in favour of dalli in 2010.

Fix merge error when Equality LHS is non-attribute

Update to activerecord-deprecated_finders on generator

Checking respond_to? incurs overhead, and most of the time when
assigning attributes it will return true. So just handle the
NoMethodError instead.

any? will check that each item in the array is truthy, as opposed to
!empty? which will simply check that the array has length. For an empty
array, !empty? still seems to be faster than any?

Rather than doing it every time an instance is instantiated.

Those z's were hard to type.

deep_dup is slow. we only need to dup the values, so just do that
directly.

Fixes #7374

Fix occasional microsecond conversion inaccuracy

This is at best a band-aid for a more proper fix, since it won't truly
handle the removal of the previous equality condition of these other
nodes. I'm planning to put in some work on ARel toward supporting that
goal.

Related: rails/arel#130, ernie/squeel#153, ernie/squeel#156

Fix slightly broken Markdown syntax in actionpack/CHANGELOG.md

I think that 5 seconds was a bit low for our purposes.

Also enable it to be configured via env vars.

We also need to scale the number of records up/down depending on how
long we're running the benchmark for.

This means we can more easily compare numbers, and we don't have to
specify a single N for all reports, which previously meant that some
tests were running many more/fewer iterations than necessary.

For consistency with the other AR extension plugins we are creating.

AM::Validation#validates: custom exception for :strict option

Conflicts:
	activemodel/CHANGELOG.md

Since we removed the ActiveModel dependenxy from ActionPack at
166dbaa7 we don't need to require it
anymore.

Closes #7370

This was reverted in https://github.com/rails/rails/commit/fb883318c8b82b3570cee022ddf6886f9052245d