Missing edge cases in Circular Buffer tests.

[julianandrews]

None of the tests for the Circular Buffer exercise use write! with a nil argument or on a buffer that isn't full. I'm assuming that in both cases write! should perform identically to write, but many simple implementations of write! are likely to get these cases wrong.

I'd be happy to add test cases to address those issues myself!

[kotp]

I would suggest doing exactly that. The tests will express exactly what it is you found, is good exercise, and makes it really simple for @kytrinyx to approve or reject or comment.

But from the words, this seems like a great set of tests to add!