spawn: fix call and promisify.

lib/utils/spawn.js

@@ -217,6 +217,23 @@ function wrap(resolve, reject) {
   };
 }
 
+/**
+ * Call a function that accepts node.js
+ * style callbacks, wrap with a promise.
+ * @private
+ * @param {Object} ctx
+ * @param {Function} func
+ * @param {Array} args
+ * @returns {Promise}
+ */
+
+function _call(ctx, func, args) {
+  return new Promise(function(resolve, reject) {
+    args.push(wrap(resolve, reject));
+    func.apply(ctx, args);
+  });
+}
+
 /**
  * Call a function that accepts node.js
  * style callbacks, wrap with a promise.
@@ -225,17 +242,13 @@ function wrap(resolve, reject) {
  */
 
 function call(func) {
-  var self = this;
   var args = new Array(Math.max(0, arguments.length - 1));
   var i;
 
   for (i = 1; i < arguments.length; i++)
     args[i - 1] = arguments[i];
 
-  return new Promise(function(resolve, reject) {
-    args.push(wrap(resolve, reject));
-    func.apply(self, args);
-  });
+  return _call(this, func, args);
 }
 
 /**
@@ -249,7 +262,13 @@ function call(func) {
 
 function promisify(func, ctx) {
   return function() {
-    return call.call(ctx, arguments);
+    var args = new Array(arguments.length);
+    var i;
+
+    for (i = 0; i < arguments.length; i++)
+      args[i] = arguments[i];
+
+    return _call(ctx || this, func, args);
   };
 }