Use isFunction to check for functionness in callFake

- Fixes #1191

src/core/SpyStrategy.js

@@ -1,4 +1,4 @@
-getJasmineRequireObj().SpyStrategy = function() {
+getJasmineRequireObj().SpyStrategy = function(j$) {
 
   function SpyStrategy(options) {
     options = options || {};
@@ -45,7 +45,7 @@ getJasmineRequireObj().SpyStrategy = function() {
     };
 
     this.callFake = function(fn) {
-      if(!(fn instanceof Function)) {
+      if(!j$.isFunction_(fn)) {
         throw new Error('Argument passed to callFake should be a function, got ' + fn);
       }
       plan = fn;

src/core/base.js

@@ -29,6 +29,10 @@ getJasmineRequireObj().base = function(j$, jasmineGlobal) {
     return j$.isA_('Number', value);
   };
 
+  j$.isFunction_ = function(value) {
+    return j$.isA_('Function', value);
+  };
+
   j$.isA_ = function(typeName, value) {
     return Object.prototype.toString.apply(value) === '[object ' + typeName + ']';
   };

src/core/requireCore.js

@@ -45,7 +45,7 @@ var getJasmineRequireObj = (function (jasmineGlobal) {
     j$.ReportDispatcher = jRequire.ReportDispatcher();
     j$.Spec = jRequire.Spec(j$);
     j$.SpyRegistry = jRequire.SpyRegistry(j$);
-    j$.SpyStrategy = jRequire.SpyStrategy();
+    j$.SpyStrategy = jRequire.SpyStrategy(j$);
     j$.StringMatching = jRequire.StringMatching(j$);
     j$.Suite = jRequire.Suite(j$);
     j$.Timer = jRequire.Timer();