util: handle symbols properly in format()

Currently, if util.format() is called with a string as its first
argument, and a Symbol as one of the subsequent arguments, an
exception is thrown due to an attempted implicit string conversion.
This commit causes Symbols to be explicitly converted.

Fixes: https://github.com/iojs/io.js/issues/927
PR-URL: https://github.com/iojs/io.js/pull/931


Reviewed-By: Domenic Denicola <domenic@domenicdenicola.com>