fix unintended breakage

We need a way to initially set the datatype, even if it should be
immutable otherwise!  This solution is kind of ugly, but I've got a
better solution in the works.

(It works as is for tagName on the controlElement because that is
currently set without going through the setAttr() interface, in the mug
constructors.)
Merge pull request #142 from dimagi/fix-type-changing

fix unintended breakage