child_process.fork: don't modify args

Fixes #1888.

lib/child_process.js

@@ -124,7 +124,7 @@ function nop() { }
 exports.fork = function(modulePath, args, options) {
   if (!options) options = {};
 
-  if (!args) args = [];
+  args = args ? args.slice(0) : [];
   args.unshift(modulePath);
 
   if (options.stdinStream) {

test/simple/test-child-process-fork.js

@@ -1,8 +1,10 @@
 var assert = require('assert');
 var common = require('../common');
 var fork = require('child_process').fork;
+var args = ['foo', 'bar'];
 
-var n = fork(common.fixturesDir + '/child-process-spawn-node.js');
+var n = fork(common.fixturesDir + '/child-process-spawn-node.js', args);
+assert.deepEqual(args, ['foo', 'bar']);
 
 var messageCount = 0;