646: toArray() fails to recognise Primary Key as DateTime

Task-Url: https://github.com/propelorm/Propel2/issues/issue/646

fixes #646

Added check for object in InstancePoolTrait to fix problem with DateTime
Object as PrimaryKey during call of save.

Changed build of $alreadyDumpedObjects in toArray method to allow DateTime
as PrimaryKey. Used serialize method in order to handle DateTime objects
used as array key.