child_process: fix spawn() optional arguments

Spawn's arguments were documented to be optional, as they are for the
other similar child_process APIs, but the code was missing. Result was
`child_process.spawn('node', {})` errored when calling slice() on an
Object, now it behaves as the documentation said it would.

lib/child_process.js

@@ -695,8 +695,16 @@ exports.execFile = function(file /* args, options, callback */) {
 };
 
 
-var spawn = exports.spawn = function(file, args, options) {
+var spawn = exports.spawn = function(file /*, args, options*/) {
-  args = args ? args.slice(0) : [];
+  var args, options;
+  if (Array.isArray(arguments[1])) {
+    args = arguments[1].slice(0);
+    options = arguments[2];
+  } else {
+    args = [];
+    options = arguments[1];
+  }
+
   args.unshift(file);
 
   var env = (options ? options.env : null) || process.env;