Keep sub-second resolution when wrapping a DateTime value

Add `DateTime#usec` and `DateTime#nsec` so that `ActiveSupport::TimeWithZone`
keeps sub-second resolution when wrapping a `DateTime` value.

Fixes #10855
Add missing nsec test for 17f5d8e
Merge pull request #10425 from ranjaykrishna/push_add_column_options_to_schema_creation

Push add column options to schema creation
Ambiguous reflections are on :through relationships are no longer supported.
For example, you need to change this:

  class Author < ActiveRecord::Base
    has_many :posts
    has_many :taggings, :through => :posts
  end

  class Post < ActiveRecord::Base
    has_one :tagging
    has_many :taggings
  end

  class Tagging < ActiveRecord::Base
  end

To this:

  class Author < ActiveRecord::Base
    has_many :posts
    has_many :taggings, :through => :posts, :source => :tagging
  end

  class Post < ActiveRecord::Base
    has_one :tagging
    has_many :taggings
  end

  class Tagging < ActiveRecord::Base
  end
Merge pull request #10890 from buytruckload/master

update format validation documetation
clean up ivar assignment
push ambiguous reflection warning down to reflection name calculation
active_record should always be set. Do or do not, there is no try
reduce automatic_inverse_of caching logic
refute the predicate for better failure messages
let the object stay in charge of internal cache invalidation
fix caching of automatic inverse of. :bomb:
oops. step away from the keyboard aaron. :cry:
Revert "Whitespace trimming in guides generation"

This reverts commit 91a1cf7013252753567b36f61bfcd5fd0a5bb2b8.

Reason: code changes disallowed in docrails, even if they are
guides generation related.
Revert "Add detailed steps on how to squash multiple commits into a single detailed commit"

This reverts commit 6df9c595ad8e473d15a81a9291e891476bc833c2.
copy edits [ci skip]
copy editing AS guide [ci skip]
Merge branch 'master' of github.com:lifo/docrails

Conflicts:
	guides/source/upgrading_ruby_on_rails.md
Merge pull request #142 from codyrobbins/master

Added a warning about #1769 to the routing guide
Don't set X-UA-Compatible header by default

We are setting this header to chrome=1 for Chrome Frame and this will be
retired soon. Check http://blog.chromium.org/2013/06/retiring-chrome-frame.html for
details