Improve the testing guide [ci skip]

Improve the "Testing Rails Applications" guide adding a section for
testing helpers and a little note about associations with ActiveRecord
and YAML fixtures.

Also add a link to Fixture Builder ; yet another replacement for
fixtures such as Machinist or FactoryGirl.
Remove conditional, since results are always an instance of `ActiveRecord::Result`
Update the filestore documentation for clear
[ci skip]
Add documentation for #clear on certain Store classes
[ci skip
Add documentation for FileStore#increment and #decrement
[ci skip]
Array#to_formatted_s does not call each element's to_s anymore

Array#to_s calls each element's inspect since ruby 1.9
Remove ambiguity with pluralizations and I18n keys used for association restrict_dependent_destroy errors
Using ruby 1.9 syntax [ci skip]
Rack Sendfile is coming as default now
Added Rack::Sendfile in rake about [ci skip]
Enhancing readability of options for delegate method
Merge branch 'master' of github.com:rails/docrails
Added some description to generate actionview gem [ci skip]
Guides fixed typo in index.html.erb file [ci skip]
New output for scaffold generate [ci skip]
fixes the test case of that plugins
Merge pull request #159 from rajcybage/my_fix

fixes the test case of that plugins
document the Rails::ConsoleMethods#helper and #controller methods

Closes rails/rails#11362
use the appropriate RDoc code markup

Thanks @fxn
use strong_params in example