[BUGFIX] Show properties of stdClass object in <f:debug>

Using fluid's <f:debug> to get dumps of json_decode()d objects
does not work currently. The DebuggerUtility shows only properties
obtainable via ReflectionClass, which does not work for stdClass objects.

The patch introduces special handling of stdClass objects, using the
properties that actually exist in the object.

Change-Id: I3845178f6defdbceae4b867f45e5080f23443766
Fixes: #60912
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/32150
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>