fix #149 Non string keys

[adrianheine]

That looks very good. Are you interested in adding some tests? Otherwise I'd merge this as it is and add tests myself.

[adrianheine]

Thanks! I'd like to have the commits squashed into one and with a commit message that's a somewhat complete phrase and starts with an upper-case letter. I also need to set up travis integration again after moving the repo to the new bublejs organization. You're code changes are good to merge from my point of view, though :)

[exe-dealer]

what about to replace String(expr) to ""+expr ? Last code is uglier but its protected from String redeclaration

[adrianheine]

I think I would prefer String, since it is prettier and performs the correct operation (toString, not valueOf).

class X {
  valueOf() { return "valueOf" }
  toString() { return "toString" }
}

console.log(
  { [new X()]: "new X()" },
  (new X()) + "",
  String(new X()),
  { valueOf: "valueOf", toString: "toString" }[new X()]
)

[adrianheine]

Ok, I merged this. Thanks for your patience!