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
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