Don't install the clock if the current timing functions aren't the originals

[finish #64116664] Fix #782

src/core/Clock.js

@@ -19,6 +19,9 @@ getJasmineRequireObj().Clock = function() {
 
 
     self.install = function() {
+      if(!originalTimingFunctionsIntact()) {
+        throw new Error('Jasmine Clock was unable to install over custom global timer functions. Is the clock already installed?');
+      }
       replace(global, fakeTimingFunctions);
       timer = fakeTimingFunctions;
       delayedFunctionScheduler = delayedFunctionSchedulerFactory();
@@ -88,6 +91,13 @@ getJasmineRequireObj().Clock = function() {
 
     return self;
 
+    function originalTimingFunctionsIntact() {
+      return global.setTimeout === realTimingFunctions.setTimeout &&
+        global.clearTimeout === realTimingFunctions.clearTimeout &&
+        global.setInterval === realTimingFunctions.setInterval &&
+        global.clearInterval === realTimingFunctions.clearInterval;
+    }
+
     function legacyIE() {
       //if these methods are polyfilled, apply will be present
       return !(realTimingFunctions.setTimeout || realTimingFunctions.setInterval).apply;