Merge branch '3.4' into 4.0

* 3.4:
  fix merge
  fixed wrong description in a phpdoc
  19 digits VISA card numbers are valid
  Add missing @ in phpdoc return statement
  Don't right trim the deprecation message
  [HttpKernel] Fixed test name
  [Debug] prevent infinite loop with faulty exception handlers
  [FrameworkBundle] fix tests
  Prefer composer install instead for using Symfony Installer
  Add the missing `enabled` session attribute
  [HttpKernel] Turn bad hosts into 400 instead of 500

Author: nicolas-grekas

Resources/config/schema/symfony-1.0.xsd

@@ -103,6 +103,7 @@
     </xsd:complexType>
 
     <xsd:complexType name="session">
+        <xsd:attribute name="enabled" type="xsd:boolean" />
         <xsd:attribute name="storage-id" type="xsd:string" />
         <xsd:attribute name="handler-id" type="xsd:string" />
         <xsd:attribute name="name" type="xsd:string" />