Coerce -0 to +0 when used in Map.set/Set.add.

This is a change in the es6 spec, rev23.
https://bugs.ecmascript.org/show_bug.cgi?id=2501#c3

Closes: #204
Merge pull request #205 from cscott/gh-204

Coerce -0 to +0 when used in Map.set/Set.add.