Fix a regression I introduced in serialization.

Although the serialization specification strongly implies that there
are no gaps in the allocated handles, it's not actually true of the RI
or -- presumably for bug-compatibility reasons -- of us. Serializing
anything with a field of type Class skips a handle; our EnumMapTest
found this.

By way of apology, I've thrown in a one-line fix for a long-standing
bug (no support for void).

Change-Id: I6104f8cb8e5a1476448ad65669e54f191c1a60e3