Stop SafeBuffer#clone_empty from issuing warnings

Logic in clone_empty method was dealing with old @dirty variable, which
has changed by @html_safe in this commit:
https://github.com/rails/rails/commit/139963c99a955520db6373343662e55f4d16dcd1

This was issuing a "not initialized variable" warning - related to:
https://github.com/rails/rails/pull/5237

The logic applied by this method is already handled by the [] override,
so there is no need to reset the variable here.

Allow empty fieldsets  to be created without a block.

Commit 3dbedd28 added NOT NULL constraints both to table
creation and modification. For creation the new default
makes sense, but the generic situation for changing a
table is that there exist records. Those records have
no creation or modification timestamps, and in the
general case you don't even know them, so when updating
a table these constraints are not going to work. See
a bug report for this use case in #3334.

Add release dates to documentation

Set "March 1, 2012" as the release date for 3.2.2, 3.1.4, 3.0.12

* master-security:
  Ensure [] respects the status of the buffer.
  delete vulnerable AS::SafeBuffer#[]
  use AS::SafeBuffer#clone_empty for flushing the output_buffer
  add AS::SafeBuffer#clone_empty
  fix output safety issue with select options

Conflicts:
	actionpack/lib/action_view/helpers/tags/base.rb

Fix layout lookup for anonymous controller

Fix for #2781

We have an empty api doc page otherwise :)

Fix warning: assigned but unused variable

remove unnecessary memcache require in ActionDispatch CacheStore

update_only documentation for accepts_nested_attributes_for

Inject in constantize

force datetime attributes to be changed, fix GH #3965

Added aliases for prev_year, prev_month, and prev_week in Time and Date calculations

The language for describing how to configure ActionController::Base.asset_host could be clearer

remove unnecessary code

This reverts commit 1b9e19cd.

it was added in 36129f21
but isn't useful anymore as corresponding tests pass without it