[HttpKernel] fixed file uploads in functional tests when no file was selected

Allow user to submit a form with no file selected.
Merge branch '2.3' into 2.4

* 2.3:
  Fixed the XPath filtering to have the same behavior than Symfony 2.4
  [DomCrawler] Fixed filterXPath() chaining
  [DomCrawler] Added more tests for the XPath filtering
  [HttpKernel] fixed file uploads in functional tests when no file was selected
  Fixed test cases failing when the Intl extension is not installed
  Fixed the Travis build to avoid exiting too early

Conflicts:
	src/Symfony/Component/DomCrawler/Crawler.php
	src/Symfony/Component/Translation/Tests/IdentityTranslatorTest.php
Merge branch '2.4'

* 2.4:
  Fixed the XPath filtering to have the same behavior than Symfony 2.4
  [DomCrawler] Fixed filterXPath() chaining
  [DomCrawler] Added more tests for the XPath filtering
  [PropertyAccess] Fixed inverted logic
  [HttpKernel] fixed file uploads in functional tests when no file was selected
  [PropertyAccess] fixed CS
  Fixed test cases failing when the Intl extension is not installed
  Fixed the Travis build to avoid exiting too early
  [PropertyAccess] Fixed getValue() when accessing non-existing indices of ArrayAccess implementations
  [PropertyAccess] Refactored PropertyAccessorCollectionTest
  [PropertyAccess] Refactored PropertyAccessorTest