Ensure (new URLSearchParams()) instanceof URLSearchParams is true, fixes

 inexorabletash#150

tests/url.js

@@ -229,6 +229,10 @@ QUnit.test('URLSearchParams', function(assert) {
   assert.equal(String(new URLSearchParams(new URLSearchParams('?a=1&b&a'))), 'a=1&b=&a=');
 });
 
+QUnit.test('instanceof URLSearchParams', function(assert) {
+  assert.ok((new URLSearchParams()) instanceof URLSearchParams);
+});
+
 QUnit.test('URLSearchParams mutation', function(assert) {
   var p = new URLSearchParams();
   assert.equal(p.get('a'), null);

url.js

@@ -460,27 +460,38 @@
     if (new global.URLSearchParams([['a', 1]]).get('a') === '1' &&
         new global.URLSearchParams({a: 1}).get('a') === '1')
       return;
+
     var orig = global.URLSearchParams;
-    global.URLSearchParams = function(init) {
+    var URLSearchParams = function(init) {
+      var o;
       if (init && typeof init === 'object' && isSequence(init)) {
-        var o = new orig();
+        o = new orig();
         toArray(init).forEach(function(e) {
           if (!isSequence(e)) throw TypeError();
           var nv = toArray(e);
           if (nv.length !== 2) throw TypeError();
           o.append(nv[0], nv[1]);
         });
-        return o;
       } else if (init && typeof init === 'object') {
         o = new orig();
         Object.keys(init).forEach(function(key) {
           o.set(key, init[key]);
         });
-        return o;
       } else {
-        return new orig(init);
+        o = new orig(init);
+      }
+      if (Object.setPrototypeOf) {
+        Object.setPrototypeOf(o, URLSearchParams.prototype);
+      } else {
+        o.__proto__ = URLSearchParams.prototype;
       }
+      return o;
     };
+
+    URLSearchParams.prototype = Object.create(orig.prototype);
+    URLSearchParams.prototype.constructor = URLSearchParams;
+
+    global.URLSearchParams = URLSearchParams;
   }());
 
 }(self));