Add support for numbers as map keys

This PR allows number to be map keys. This was originally thought to have been fixed by https://github.com/sass/libsass/pull/596 but it wasn't.

Fixes https://github.com/sass/libsass/issues/534. Spec added https://github.com/sass/sass-spec/commit/6ec734b37cb50927846264176d631d9084913672, https://github.com/sass/sass-spec/pull/138.