hexbin: Fixed incorrect "optional" attribute. Defined default behavior.
`x` and `y` have no default values and are thus not optional.

The semantics of hexbin() with default arguments (i.e. when only `x` and `y` are given) is now defined.

It would be nice to describe what kind of argument `reduce_C_function()` expects (A sequence? A Series?), but this is not immediately obvious from the code.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
