Save has_one associations only if record has changes

Prevents save related callbacks such as `after_commit` being
triggered when `has_one` objects are already persisted and have no
changes.
Replace map.flatten with flat_map in actionpack
Replace map.flatten with flat_map in activerecord
Replace map.flatten with flat_map in railties
Fix OrderedHash.select to return self instance.

On ruby 2.1.1 the behavior of .select and .reject has changed.
They will return a Hash new instance, so we need to override them to
keep the instance object class.
Make the rails:template rake task load initializers

Templates could rely on irregular inflections or external libraries for
instance so we should load the application's initializers when running
the rails:template task.

The introducing commit of this feature is f7f11361 ; the initializers
have never been loaded invoking this task.

Fixes #12133.
Merge pull request #14244 from arthurnn/orderd_hash_select_fix

Fix OrderedHash.select to return self instance.
Merge pull request #14325 from robin850/issue-12133

Make the rails:template rake task load initializers
Fix errors for four of the code samples
The four code samples that fail to run are:

- Add attribute magic to objects. Fixed by introducing a Person
  instance variable.

- Tracking value changes. Fixed by replacing `attr_accessor` with
  `define_attribute_methods`, providing getter and setter methods
  for `name` and providing the missing `Person#save` method. A
  call to `Person#save` has to precede the `person.name = 'robert'`
  assignment, if we want `previous_changes` to include 'bob'.

- Adding `errors` interface to objects. Fixed by introducing a
  Person instance variable, assigning `nil` to its name and calling
  `Person#validate!`.

- Custom validators. Fixed by defining `HasNameValidator` before
  it is used by `ValidatorPerson`.

All the code samples can now be run smoothly.

Call Dirty#changes_applied in Person#save, instead of modifying instance vars.
Merge pull request #14313 from m-Peter/active_model_doc_fixes

Fix errors for four of the code samples [ci skip]
Move setting :scope_level_resource to resource_scope

Originally with_scope_level was exclusively for managing scope levels with
resources, however it is now used for other things so it makes more sense
to move the responsibility for setting the :scope_level_resource to the
resource_scope method. This eliminates repeatedly setting it to the same
resource as each resource method scope is evaluated.
Only use shallow nested scope when depth is > 1

By tracking the depth of resource nesting we can push the need for nested
shallow scoping to only those routes that are nested more than one deep.
This allows us to keep the fix for #12498 and fix the regression in #14224.

Fixes #14224.
Pull namespace defaults out of the options hash

If a developer has specified either :path or :as in the options hash then
these should be used as the defaults for :shallow_path and :shallow_prefix.

Fixes #14241.
Copy shallow options from normal options when using scope

If the options :shallow_prefix and :shallow_path are not set in the
scope options then copy them from the normal :as and :path options
if they are set.
Move changelog entry to the top, highlight module name [ci skip]
Merge pull request #14262 from sferik/flat_map

Replace additional instances of map.flatten with flat_map
Run ActiveModel test suites in random order.

This gets the whole ActiveModel test suites working even if
`self.i_suck_and_my_tests_are_order_dependent!` is disabled
in `ActiveSupport::TestCase`.

Two places are found that potentially leak global state. This patch
makes sure states are restored so that none of the changes happen in
a single test will be carried over to subsequence tests.
Merge pull request #14315 from zuhao/activemodel_tests_in_random_order

Run ActiveModel test suites in random order.
Enhance readability of ActionView DateHelper#distance_of_time_in_words

Refactor numerical constants to module constants which give the numbers
a contextual meaning. This commit aims to provide quicker understanding
for part of the implementation of the
DateHelper#distance_of_time_in_words method.
Merge pull request #14334 from kennym/master

Enhance readability of ActionView DateHelper#distance_of_time_in_words