* Function to clear the DO bit from an OPT RR.
* Tests for ClearDo() function.
* Changed from ClearDo() to SetDo() with an optional argument.
* Update doc string for SetDo(). Make tests for SetDo() comprehensive.
We were off by a value of 15. This fixes it. Hard to come up with a test
as writing and reading it yourself will be consistent.
Don't allows extended rcodes smaller than 16. And fix the tests as well.