Add merge support for PSCustomObject (and array of..)

[gaelcolas]

The current version only supports merging Hashtable, array of hashtables, and basic types.
PSCustomObjects or arrays of PSCustomObjects would throw an exception or silently ignore the required merges.

This is making impossible to merge data that spans across files (as a File/Folder objects returns PSCustomObject by the Datum File Provider), so you can't split a Role as a folder with keys as Files.

The Merged data should be a hashtable, ready to be used for splatting.

[gaelcolas]

As everything is now casted to an ordered Hashtable, that is 'fixed'