Fix default value handling for multi-value options

The default value for array options will be an array, so it is not suitable to use as the default when processing one of many values for a multi-value option. Using null seems appropriate here, as it indicates the absence of a value and also converts nicely to an empty string (as opposed to an empty array).

Fixes #7689
Merge branch '2.1' into 2.2

* 2.1:
  Fix default value handling for multi-value options
  [HttpKernel] truncate profiler token to 6 chars (see #7665)
  Disabled APC on Travis for PHP 5.5+ as it is not available
  [HttpFoundation] do not use server variable PATH_INFO because it is already decoded and thus symfony is fragile to double encoding of the path
  [Yaml] improved boolean naming ($notEOF -> !$EOF)
  [Yaml] fixed handling an empty value
  [Routing][XML Loader] Add a possibility to set a default value to null
  The /e modifier for preg_replace() is deprecated in PHP 5.5; replace with preg_replace_callback()
  [HttpFoundation] Fixed bug in key searching for NamespacedAttributeBag
  [Form] DateTimeToRfc3339Transformer use proper transformation exteption in reverse transformation
  Update PhpEngine.php
  [HttpFoundation] getClientIp is fixed.

Conflicts:
	.travis.yml
	src/Symfony/Component/Routing/Loader/XmlFileLoader.php
	src/Symfony/Component/Routing/Loader/schema/routing/routing-1.0.xsd
	src/Symfony/Component/Routing/Tests/Fixtures/validpattern.xml
	src/Symfony/Component/Routing/Tests/Loader/XmlFileLoaderTest.php