Read StringIO with foreach

[MarcPer]

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.

Current behavior

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)

[kou]

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

[MarcPer]

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