[VarDumper] fix and test PdoCaster
Merge branch '2.6' into 2.7

* 2.6:
  Add LegacyPdoSessionHandler class
  fix dependency on ExtensionInterface over implementation
  [FrameworkBundle] forward error reporting level to insulated Client
  [VarDumper] fix and test PdoCaster
  [Config] fix error handler restoration in test