feat(spy): add option to respy

will not throw error if spyOn called on a spy

fixes #931

src/core/Env.js

@@ -270,6 +270,10 @@ getJasmineRequireObj().Env = function(j$) {
       return runnableResources[currentRunnable().id].spies;
     }});
 
+    this.allowRespy = function( allow ){
+      spyRegistry.setAllowRespy( allow );
+    };
+
     this.spyOn = function() {
       return spyRegistry.spyOn.apply(spyRegistry, arguments);
     };