Bugfix for view context cache picking methods from Object.prototype instead of the view.

Fixes #442

mustache.js

@@ -356,7 +356,7 @@
     var cache = this.cache;
 
     var value;
-    if (name in cache) {
+    if (cache.hasOwnProperty(name)) {
       value = cache[name];
     } else {
       var context = this, names, index, lookupHit = false;

test/_files/avoids_obj_prototype_in_view_cache.js

@@ -0,0 +1,4 @@
+({
+  valueOf: 'Avoids methods',
+  watch: 'in Object.prototype'
+})

test/_files/avoids_obj_prototype_in_view_cache.mustache

@@ -0,0 +1 @@
+{{valueOf}} {{watch}}

test/_files/avoids_obj_prototype_in_view_cache.txt

@@ -0,0 +1 @@
+Avoids methods in Object.prototype