Make QueryString.parse run faster

Use decodeURIComponent when appropriate, and only fall back to
querystring.decode if it throws, or if the character is a '+'.

Fix nodejs#2248

lib/querystring.js

@@ -170,9 +170,17 @@ QueryString.parse = QueryString.decode = function(qs, sep, eq) {
   }
 
   qs.split(sep).forEach(function(kvp) {
-    var x = kvp.split(eq);
-    var k = QueryString.unescape(x[0], true);
-    var v = QueryString.unescape(x.slice(1).join(eq), true);
+    var x = kvp.split(eq), k, v, useQS=false;
+    try {
+      if (kvp.match(/\+/)) {	// decodeURIComponent does not decode + to space
+        throw "has +";
+      }
+      k = decodeURIComponent(x[0]);
+      v = decodeURIComponent(x.slice(1).join(eq) || "");
+    } catch(e) {
+      k = QueryString.unescape(x[0], true);
+      v = QueryString.unescape(x.slice(1).join(eq), true);
+    }
 
     if (!hasOwnProperty(obj, k)) {
       obj[k] = v;