Improve img alt attribute for screen readers

Currently, the img_alt method in ActionView keeps underscores
in the alt attribute. Because underscores are pronounced in
Apple's VoiceOver Utility, this has serious implications for
accessibility. This patch makes underscored or hyphenated file
names (both common in projects) read more naturally in screen
readers by replacing them with spaces. See method documentation
for details.

Added documentation to image_alt method
Supress warning about method redifinition

In i18n gem, the following methods are defined.
- `Hash#except`
- `Hash#slice`

But if there are defined already, i18n skips these definitions.
So these definition by `active_support` are required before `require 'i18n'`.
Merge pull request #9240 from tricknotes/supress-warning

Supress warning about method redifinition
fix a typo on the engine guide
Merge pull request #9245 from vasconcelloslf/fix_guide_typo

fix a typo on the engine guide
fixing bit string test
Merge pull request #9221 from thenickcox/image_alt_attribute

Improve img alt attribute for screen readers
Add changelog entry for #9177 [ci skip]

Add test:all and test:all:db rake tests to speed up full test runs.
Improve changelog entry from #9221 [ci skip]
define Active Record Store accessors in a module

Allow store accessors to be overrided like other attribute methods,
e.g.:

    class User < ActiveRecord::Base
      store :settings, accessors: [ :color, :homepage ], coder: JSON

      def color
        super || 'red'
      end
    end
remove trailing whitespace from engines guide
fix the example code in the engines guide. closes #8583.
Merge pull request #9247 from lest/store-module

define Active Record Store accessors in a module
Merge pull request #9250 from senny/8583_fix_engines_documentation_example_code

fix example code in engines guide
test for regression from a712e08ebe21f6d8653a0e6602df2e0f5d40d9ca