* Short-circuit SerializeGroupedNodes on no child nodes.
Avoids creating unused Dictionary.
* Don't create Dictionary in ReadAttributeElements if it'll be empty.
* Reduce calls to JsonReader.TokenType virtual property.
* Remove duplicate finished boolean.
Different reasons for finishing the ReadAttributeElements loop are
noted, but the difference isn't acted on, so merge them.
* Check correct XML is from correct TokenType immediately.
(This changes the exception message in the case of it also being an
invalid objectType, but not the exception type).
Jon Hanna
b3e2d61708