Fix Baseclass becomes! subclass.
Fixed `ActiveSupport::TimeWithZone#-` so precision is not unnecessarily lost

When working with objects with a nanosecond component, the `-` method may
unnecessarily cause loss of precision.

`ActiveSupport::TimeWithZone#-` should return the same result as if we were
using `Time#-`:

    Time.now.end_of_day - Time.now.beginning_of_day #=> 86399.999999999

Before:

    Time.zone.now.end_of_day.nsec #=> 999999999
    Time.zone.now.end_of_day - Time.zone.now.beginning_of_day #=> 86400.0

After:

    Time.zone.now.end_of_day - Time.zone.now.beginning_of_day
    #=> 86399.999999999
Test the serialized types of virtual columns in XML

The previous tests were passing, because nothing ever looked at the
generated XML. What was previously being generated was
`<firstname type="NilClass">...`, which is not consistent with all other
cases where there is not a known type.
Refactor XML serializer to not depend on `serialized_attributes`
[ci skip] fix "note" syntax
[ci skip] less derogatory explanation of defaults
[ci skip] add fixture docs link to guides

The fixtures API docs are really comprehensive. We should either be driving people there, or perhaps making fixtures its own guide.
[ci skip] explain association behavior. 

When you manually specify `id:` attribute in a fixture Rails' built in association assignment feature will no longer work. http://stackoverflow.com/questions/23985675/incorrect-association-in-fixtures

This commit raises awareness of this behavior and encourages developers to look directly at the fixture documentation.
Fix regression on eager loading association based on SQL query rather
than existing column.

Fixes #15480.
Add default_i18n_subject to the guides

[ci skip]
Pluralize params
push splitting "to" up the callstack
Convert StrongParameters cache to a hash. This fixes an unbounded
memory leak demonstrated on @tenderlove's latest blog post:

http://tenderlovemaking.com/2014/06/02/yagni-methods-are-killing-me.html
reuse available belongs_to? method

Reflection has a `belongs_to?` method. Instead of checking for
`macro == :belongs_to` throughout the source reuse existing
method.

I also bumped `foreign_key_present?` method onto on line because
the `belongs_to?` makes it shorter than other longer lines in
the same class.
Merge pull request #15499 from eileencodes/reuse-available-belongs_to-method

reuse available belongs_to? method
Merge pull request #15498 from zenspider/fix_memory_leak

Convert StrongParameters cache to a hash. This fixes an unbounded memory leak
Cleaned up duplicated CHANGELOG entry [ci skip]
Change wording of explanation about precision & scale of decimal numbers [ci skip]
Merge pull request #15482 from laurocaetano/fix-regression-for-eager-load

Fix regression on eager loading association based on SQL query rather than existing column.

Conflicts:
	activerecord/CHANGELOG.md
Merge pull request #15500 from prathamesh-sonpatki/change-wording-of-explanation-about-precision-and-scale-of-decimal-numbers

Change wording of explanation about precision & scale of decimal numbers [ci skip]