Fix for CBL-ios issue 648

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This is my fix for #648. Intentionally excluding NSDate in the NSObject subclass check since it is handled best in the final else catch-all case, and NSObject handing of NSDate is not appropriate/does not work. Maybe more elegant way to implement this as there might be other cases (e.g. other classes) that the coe should be letting the final catch-all else handle using ValueConverter handle (I doubt NSDate is the only such case?), but wanted to avoid significantly changing the structure/flow of this code as it is not originally my code and my understanding of it is not good enough and wanted to leave the least trace possible with this fix, I can imagine reworking this in a few ways to handle this, but it would probably change the method and break other cases, so excluding NSDate subclasses in the NSObject else if I added for now...

#648

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To add on to my above comments on the nature of excluding certain classes in the NSObject subclass check (which I think is a bit kludgy), perhaps a cleaner way of accomplishing this is to, in the final catch-all else (in both array item and property code paths) to try value converter first and if converter is nil, then do return [super impForGetterOfProperty: property ofClass: propertyClass];, would do a better job of "deferring to ValueConverter" like I mention above than the current implementation, perhaps?

Also make updates to ValueConverter to handle more data types "automatic" in this case....

[24601]

Implemented the above fix as it is much cleaner and will resubmit this PR.