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
[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.
Bring type casting behavior of hstore/json in line with serialized

`@raw_attributes` should not contain the type-cast, mutable version of
the value.
[ci skip] remove double "in your"
Removed CaptureHelper#flush_output_buffer as it is only used in tests.
reviewed: @tenderlove
Fix inverse associations test

`InverseHasManyTests#test_parent_instance_should_be_shared_within_create_block_of_new_child`
was mistakenly the same as
`InverseHasManyTests#test_parent_instance_should_be_shared_within_build_block_of_new_child`.
Update getting_started.md

It may be confusing to a Rails newbie why "Article" is suddenly being referred to with the "A" being capitalized. Thus far in the guide, the "a" has always been in lowercase. I think it is helpful to point out the reason for this change (especially since not using the capital "A" will result in an error). [ci skip]
Update getting_started.md

It's not very clear exactly what the "params" method is and does, as the current description in the guide is somewhat vague and brief. An example is helpful, especially considering this method is referenced over 30 times in this guide. [ci skip]
Merge pull request #15515 from leafac/fix-inverse-associations-block-test

Fix inverse associations test
Merge pull request #15518 from jonathancadepowers/master

Added Two Clarification "Tips" [ci skip]
Merge pull request #15421 from gchan/time_with_zone_precision

Fixed `ActiveSupport::TimeWithZone#-` so precision is not unnecessarily lost
Remove deprecated method ActiveRecord::Migrator.proper_table_name
Merge pull request #15512 from akshay-vishnoi/delete-deprecated-method

Remove deprecated method ActiveRecord::Migrator.proper_table_name
Fix behavior of handling BC era dates.

BC era year is (astronomical year + 1) and starts from 1 BC.
document include_blank's usage with a string [ci skip]
Avoid hardcoded value in test setup/teardown.