Merge branch '2.8' into 3.4

* 2.8:
  KernelInterface can return null container
  [Ldap] Use shut up operator on connection errors at ldap_start_tls
  [HttpFoundation] don't override StreamedResponse::setNotModified()
  Added relevent links for parsing to the phpdoc
  Add stricter checking for valid date time string
  [Form] Fix DateTimeType html5 input format

Author: nicolas-grekas

Tests/Extension/AbstractBootstrap3LayoutTest.php

@@ -1560,7 +1560,7 @@ public function testDateTimeWithWidgetSingleText()
     [@type="datetime-local"]
     [@name="name"]
     [@class="my&class form-control"]
-    [@value="2011-02-03T04:05:06Z"]
+    [@value="2011-02-03T04:05:06"]
 '
         );
     }
@@ -1581,7 +1581,7 @@ public function testDateTimeWithWidgetSingleTextIgnoreDateAndTimeWidgets()
     [@type="datetime-local"]
     [@name="name"]
     [@class="my&class form-control"]
-    [@value="2011-02-03T04:05:06Z"]
+    [@value="2011-02-03T04:05:06"]
 '
         );
     }