feat(spy): add option to respy

will not throw error if spyOn called on a spy

fixes #931

spec/core/integration/EnvSpec.js

@@ -677,6 +677,35 @@ describe("Env integration", function() {
     env.execute();
   });
 
+  it('allows to respy functions', function () {
+
+    var env = new j$.Env(),
+        foo = {
+          'bar': function () {
+            return 1;
+          }
+        };
+
+    env.allowRespy(true);
+
+    env.describe('test suite', function(){
+        env.it('spec 0', function(){
+          env.spyOn(foo,'bar');
+
+          var error = null;
+
+          try {
+            env.spyOn(foo, 'bar'); // no exception thrown
+          }catch(e){
+            error = e;
+          }
+          expect(error).toBeNull();
+        });
+    });
+
+    env.execute();
+  });
+
   it('removes all spies added in a spec after the spec is complete', function(done) {
     var env = new j$.Env(),
       originalFoo = function() {},

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);
     };

src/core/SpyRegistry.js

@@ -4,6 +4,10 @@ getJasmineRequireObj().SpyRegistry = function(j$) {
     options = options || {};
     var currentSpies = options.currentSpies || function() { return []; };
 
+    this.setAllowRespy = function( allow ){
+      this.respy = allow;
+    };
+
     this.spyOn = function(obj, methodName) {
       if (j$.util.isUndefined(obj)) {
         throw new Error('spyOn could not find an object to spy upon for ' + methodName + '()');
@@ -17,9 +21,12 @@ getJasmineRequireObj().SpyRegistry = function(j$) {
         throw new Error(methodName + '() method does not exist');
       }
 
-      if (obj[methodName] && j$.isSpy(obj[methodName])) {
-        //TODO?: should this return the current spy? Downside: may cause user confusion about spy state
-        throw new Error(methodName + ' has already been spied upon');
+      if (obj[methodName] && j$.isSpy(obj[methodName])  ) {
+        if ( !!this.respy ){
+          return obj[methodName];
+        }else {
+          throw new Error(methodName + ' has already been spied upon');
+        }
       }
 
       var spy = j$.createSpy(methodName, obj[methodName]);