Read StringIO with foreach

Proposal

I'd like to propose being able to open a StringIO s with CSV.foreach(s), just as it's possible to open a file with CSV.foreach(path).

I can investigate and open a PR if this behavior is desired.

The documentation already says it should be possible to pass an IO object to foreach, but if an instance of StringIO is passed, we get:

no implicit conversion of StringIO into String (TypeError)

If CSV.foeach(io) works, CSV.foreach(string_io) should work too.

Great, I'll open a PR with proposed changes in the next days.