Further refactor build_conditions in route set

Return the conditions from the keep_if call, and ignore the value
argument since it's not being used.
check for nil or empty record in form_for

if nil or an empty array is passed into form_for you get a horrible error message, this one is much more indicative of what the programmer needs to know to fix the problem.
Merge pull request #7329 from guilleiguaran/move-default-headers-ad-railtie

Move AD default_headers configurations to railtie
Merge pull request #7314 from schneems/schneems/form_for_error

Check for Blank Record in form_for
Bring back changelog entries for Active Support

Removed in 0228a73b1094a3e19ad291d2ce4789890c09578a, pull request #7310.
Fallback to :en locale instead of handling a constant with defaults

Action Pack already comes with a default locale fine for :en, that is
always loaded. We can just fallback to this locale for defaults, if
values for the current locale cannot be found.

Closes #4420, #2802, #2890.
Ensure I18n format values always have precedence over defaults

Always merge I18n format values, namespaced or not, over the default
ones, to ensure I18n format defaults will have precedence over our
namespaced values.

Precedence should happen like this:

    default :format
    default :namespace :format
    i18n    :format
    i18n    :namespace :format

Because we cannot allow our namespaced default to override a I18n
:format config - ie precision in I18n :format should always have higher
precedence than our default precision for a particular :namespace.

Also simplify default format options logic.
Merge branch 'number-helpers-defaults'

Closes #6290
[guides] Add info about CHANGELOGs to contributing guide
Added changelog entry for table name prefix fix [ci skip]
Merge pull request #7331 from morgoth/changelog-for-engine-table-name-prefix-generator-fix

Added changelog entry for table name prefix fix [ci skip]
Add CHANGELOG entry for #7314
Sync CHANGELOGs [ci skip]
Refactor model name logic and make it a real private class method

Under a "private" call, class methods are not real private methods,
they're public just like any other method.

Make model_name_from_record_or_class a private class method, and nodoc it.
Simplify html attributes generation for options_for_select

Further simplify the option_html_attributes method after the changes
introduced in dacbcbe55745aa9e5484b10b11f65ccca7db1c54 to not escape the
html options here (since they're going to be escaped down the chain in
content tag).
Ensure option_html_attributes does not modify the given option hashes

We can avoid creating extra hashes with #merge, and use #merge! instead.
Evented notifications take priority over Timed notifications

In cases where a notification subscriber includes methods to support
both Evented and Timed events, Evented should take priority over Timed.
This allows subscribers to be backwards compatible (older Rails only
allows Timed events) while defaulting to newer behavior.
Restore state on create when ActiveRecord::RecordInvalid is raised

This fixes issue #3217.
Merge pull request #6073 from daveyeu/restore-state-on-record-invalid

Restore state on create when ActiveRecord::RecordInvalid is raised
Missing closing tag