Don't print non-enumerable symbols

[novemberborn]

When printing objects, only enumerable property names are printed. This seems reasonable. However, all property symbols are printed, whether they're enumerable or not.

As suggested by @caesarsol in #2 (comment).

[vadimdemedes]

I agree, if it doesn't affect assertion, it shouldn't be printed. They aren't enumerable, so it's kind of by definition that we shouldn't list them.