add the bare and separator-only variants of addString to Iterator

[tim-ruhland]

addresses #401

[tim-ruhland]

Since this is my first pull request, I am confirming that I have signed Scala CLA.

[julienrf]

What do you think of using default parameter values instead of overloading?

[tim-ruhland]

Keeping the overloaded functions may be better than default parameter values because using default parameters would change the usage for second variant from addString(b, sep) to addString(b, "", sep) and that would not be preferable. Also, the overloaded methods follow the mkString variants, and whatever approach is decided should be applied to both.

[julienrf]

Thanks, can you rebase your work and fix conflicts?