Fix handling SCRIPT_NAME from within mounted engine's

When you mount your application at a path, for example /myapp, server
should set SCRIPT_NAME to /myapp. With such information, rails
application knows that it's mounted at /myapp path and it should generate
routes relative to that path.

Before this patch, rails handled SCRIPT_NAME correctly only for regular
apps, but it failed to do it for mounted engines. The solution was to
hardcode default_url_options[:script_name], which is not the best answer
- it will work only when application is mounted at a fixed path.

This patch fixes the situation by respecting original value of
SCRIPT_NAME when generating application's routes from engine and the
other way round - when you generate engine's routes from application.

This is done by using one of 2 pieces of information in env - current
SCRIPT_NAME or SCRIPT_NAME for a corresponding router. This is because
we have 2 cases to handle:

- generating engine's route from application: in this situation
  SCRIPT_NAME is basically SCRIPT_NAME set by the server and it
  indicates the place where application is mounted, so we can just pass
  it as :original_script_name in url_options. :original_script_name is
  used because if we use :script_name, router will ignore generating
  prefix for engine

- generating application's route from engine: in this situation we
  already lost information about the SCRIPT_NAME that server used. For
  example if application is mounted at /myapp and engine is mounted at
  /blog, at this point SCRIPT_NAME is equal /myapp/blog. Because of that
  we need to keep reference to /myapp SCRIPT_NAME by binding it to the
  current router. Later on we can extract it and use when generating url

Please note that starting from now you *should not* use
default_url_options[:script_name] explicitly if your server already
passes correct SCRIPT_NAME to rack env.

(closes #6933)
Use method compilation for association methods

Method compilation provides better performance and I think the code
comes out cleaner as well.

A knock on effect is that methods that get redefined produce warnings. I
think this is a good thing. I had to deal with a bunch of warnings
coming from our tests, though.
fixup guides in light of :dependent changes
Move AD default_headers configurations to railtie

ActionDispatch railtie is a better place for
config.action_dispatch.default_headers settings, users can continue
overriding those settings in their configuration files if needed.
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.
Add doc for config.action_dispatch.default_headers

Add documentation for config.action_dispatch.default_headers to
Rails Configuring guide.
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).