JsonUnsupportedObjectError thrown for DateTime and Uri classes when using jsonEncode

[bruceevans]

Was attempting to dump the book query info to json locally for future use, kept hitting this error: JsonUnsupportedObjectError. I was able to get around it by converting the 'publishedDate' DateTime to a string and not serializing anything with a link.

Seems these two classes don't have a proper toJson method, might want to grab their string representation.