Overriding spaces_ shouldn't be possible

[pdubroy]

Right now it's possible to override spaces_, even though there's a comment in the code saying that it shouldn't be possible to override it.

See http://jsfiddle.net/pdubroy/seuwpew3/.

[alexwarth]

The fix for this would be to create a separate rule that's used just for property names (in object patterns) and modify name so that it no longer accepts '_'s, but IMO it's not worth it -- the add'l complexity in the Ohm grammar outweighs the benefits of this change, especially as the name spaces_ already discourages programmers from messing with this rule.

Plus, if we ever do decide to disallow '_'s in rule names, inline rule declarations won't be a syntactic sugar anymore. (Well, to be honest, they already aren't, b/c they are handled specially when they show up inside a parameterized rule.)

Anyway, here's a change that makes this comment true: a65ac62