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: #927
PR-URL: #931
Reviewed-By: Domenic Denicola <domenic@domenicdenicola.com>

lib/util.js

@@ -30,7 +30,7 @@ exports.format = function(f) {
     }
   });
   for (var x = args[i]; i < len; x = args[++i]) {
-    if (x === null || typeof x !== 'object') {
+    if (x === null || (typeof x !== 'object' && typeof x !== 'symbol')) {
       str += ' ' + x;
     } else {
       str += ' ' + inspect(x);

test/parallel/test-util-format.js

@@ -1,6 +1,7 @@
 var common = require('../common');
 var assert = require('assert');
 var util = require('util');
+var symbol = Symbol('foo');
 
 assert.equal(util.format(), '');
 assert.equal(util.format(''), '');
@@ -14,6 +15,15 @@ assert.equal(util.format('test'), 'test');
 // CHECKME this is for console.log() compatibility - but is it *right*?
 assert.equal(util.format('foo', 'bar', 'baz'), 'foo bar baz');
 
+// ES6 Symbol handling
+assert.equal(util.format(symbol), 'Symbol(foo)');
+assert.equal(util.format('foo', symbol), 'foo Symbol(foo)');
+assert.equal(util.format('%s', symbol), 'Symbol(foo)');
+assert.equal(util.format('%j', symbol), 'undefined');
+assert.throws(function() {
+  util.format('%d', symbol);
+}, TypeError);
+
 assert.equal(util.format('%d', 42.0), '42');
 assert.equal(util.format('%d', 42), '42');
 assert.equal(util.format('%s', 42), '42');