Don't use instanceof to determine if a Context is a Context

Fixes #743

lib/dust.js

@@ -328,12 +328,17 @@
     this.options = options;
     this.blocks = blocks;
     this.templateName = templateName;
+    this._isContext = true;
   }
 
   dust.makeBase = dust.context = function(global, options) {
     return new Context(undefined, global, options);
   };
 
+  dust.isContext = function(obj) {
+    return typeof obj === "object" && obj._isContext === true;
+  };
+
   /**
    * Factory function that creates a closure scope around a Thenable-callback.
    * Returns a function that can be passed to a Thenable that will resume a
@@ -347,7 +352,7 @@
   }
 
   Context.wrap = function(context, name) {
-    if (context instanceof Context) {
+    if (dust.isContext(context)) {
       context.templateName = name;
       return context;
     }

test/core.spec.js

@@ -72,6 +72,14 @@
         expect(base.options.lang).toEqual(opts.lang);
       });
     });
+
+    describe('prototype', function() {
+      var base = dust.context({
+        sayHello: function() { return "Hello!"; }
+      }).push({ foo: 'bar' });
+      var context = extend({}, base);
+      renderIt('survives having its prototype destroyed', '{sayHello} {foo}', context, 'Hello! bar');
+    });
   });
 
   it("valid keys", function() {