Fix issue with set readonly throwing an exception

[arontsang]

No description provided.

[arontsang]

fixes #83

[nbudin]

I have somewhat mixed feelings about this, because it feels like making the update behavior dependent on readOnly is non-obvious and could backfire in surprising ways. But, on the other hand, the current behavior is also surprising, so I think this is a good idea and I'll merge it.

This is sort of in the gray area between "breaking" and "non-breaking" change. My thought is that it's unlikely to break any actual use of this API, so I'm going to make this a minor version bump.

Thanks for the PR!