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

`@raw_attributes` should not contain the type-cast, mutable version of
the value.
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.
Merge pull request #15525 from zuhao/refactor_actionpack_query_string_parsing_test

Avoid hardcoded value in test setup/teardown.
Mime::PNG is already defined.
Merge pull request #15521 from edogawaconan/fix_bc_postgres_master

Fix BC year handling in postgres
Merge pull request #15529 from zuhao/refactor_actionpack_mime_type_test

Make sure Mime::Type is not altered after tests.
Reset callbacks after test.

Otherwise the state of callback chain is leaked.