Merge branch '2.7' into 2.8

* 2.7:
  [HttpFoundation] Use the correct syntax for session gc based on Pdo driver
  Revert "bug #24987 [Console] Fix global console flag when used in chain (Simperfit)"
  Revert "bug #25487 [Console] Fix a bug when passing a letter that could be an alias (Simperfit)"
  Disable CSP header on exception pages only in debug
  Fixed submitting disabled buttons
  Fixed Button::setParent() when already submitted
  Improve assertions
  SCA: get rid of repetitive calls
  allow null values for root nodes in YAML configs
  [VarDumper] Fix docblock
  Improve phpdoc to make it more explicit

Author: fabpot

ExecutionContext.php

@@ -197,8 +197,9 @@ public function validate($value, $subPath = '', $groups = null, $traverse = fals
     {
         $propertyPath = $this->getPropertyPath($subPath);
 
+        $visitor = $this->globalContext->getVisitor();
         foreach ($this->resolveGroups($groups) as $group) {
-            $this->globalContext->getVisitor()->validate($value, $group, $propertyPath, $traverse, $deep);
+            $visitor->validate($value, $group, $propertyPath, $traverse, $deep);
         }
     }