Fixed the handling of boolean attributes in ChoiceFormField

An option is marked as selected by the presence of the selected attribute,
not by the presence of a non-empty selected attribute. The same is true
for checked radio buttons or checkboxes.
[DomCrawler] Fixed the initial state for options without value attribute
bug #10926 [DomCrawler] Fixed the initial state for options without value attribute (stof)

This PR was merged into the 2.3 branch.

Discussion
----------

[DomCrawler] Fixed the initial state for options without value attribute

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | n/a

Commits
-------

78cff96 [DomCrawler] Fixed the initial state for options without value attribute
[DomCrawler] Fixed the coding standards to use strict comparisons
Changed the default value of checkbox and radio to match the HTML spec

When the checkbox or radio input does not have a value attribute, the HTML
spec defines that the value should be 'on', not '1'.
Merge branch '2.3' into 2.4

* 2.3:
  [HttpFoundation] implement session locking for PDO
  [DomCrawler] Fixed the coding standards to use strict comparisons
  [HttpKernel] removed absolute paths from the generated container
  [DomCrawler] Fixed the initial state for options without value attribute
  Fixed the handling of boolean attributes in ChoiceFormField

Conflicts:
	src/Symfony/Component/DomCrawler/Tests/Field/ChoiceFormFieldTest.php
	src/Symfony/Component/HttpKernel/Tests/KernelTest.php
Merge branch '2.4'

* 2.4:
  [HttpFoundation] implement session locking for PDO
  [DomCrawler] Fixed the coding standards to use strict comparisons
  [HttpKernel] removed absolute paths from the generated container
  [DomCrawler] Fixed the initial state for options without value attribute
  Fixed the handling of boolean attributes in ChoiceFormField
  Fixed FormDataExtractorTest_SimpleValueExporter::exportValue not implementing the interface correctly
  Updated form dev dependencies
bug #10933 Changed the default value of checkbox and radio to match the HTML spec (stof)

This PR was merged into the 2.5-dev branch.

Discussion
----------

Changed the default value of checkbox and radio to match the HTML spec

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | n/a

When the checkbox or radio input does not have a value attribute, the [HTML spec](http://www.whatwg.org/specs/web-apps/current-work/multipage/states-of-the-type-attribute.html#checkbox-state-%28type=checkbox%29) defines that the value [should be ``on``](http://www.whatwg.org/specs/web-apps/current-work/multipage/common-input-element-attributes.html#dom-input-value-default-on), not ``1``.

This change is a small BC break as a testsuite with an assertion on the ``1`` value would fail after the change. However, this means that such testsuite would assert a different behavior than the real usage of the application.
Note that people using the DomCrawler component on pages using the Symfony Form component are not affected by the change as the Form component always set an explicit value on checkboxes and radio buttons

Commits
-------

186b65e Changed the default value of checkbox and radio to match the HTML spec