Add spec for StringIO#getch

This method is added by io/console on require.
Include IO::{Writable,Readable} in StringIO. Fixes #2348

io/console reopens IO::Readable to add #getch, so we need to include it in
StringIO as well. Although we mimic MRI here, a nicer thing would've been
probably make io/console add #getch into StringIO directly rather than through
the module.