Fixed so that timing will always use native Date instance instead of …

…mock even for badly designed tests (#240)

The user might mock the Date. Because of this KarmaReporter saved a reference to the native Date and used that reference for measuring duration. The problem was that it only worked when users followed best practises when writing tests. 
If a user wrote a "bad" test where the Date is mocked immediately in the body of a decribe instead of inside an it or beforeEach, the Date was already mocked by the time KarmaReporter was called. By saving the reference at the time adapter.js is executed instead of when KarmaReporter is run, this problem is circumvented.

src/adapter.js

@@ -2,6 +2,10 @@
 
 'use strict'
 
+// Save link to native Date object
+// before it might be mocked by the user
+var _Date = Date
+
 /**
  * Decision maker for whether a stack entry is considered external to jasmine and karma.
  * @param  {String}  entry Error stack entry.
@@ -142,9 +146,6 @@ function getAllSpecNames (topSuite) {
 function KarmaReporter (tc, jasmineEnv) {
   var currentSuite = new SuiteNode()
 
-  // Save link on native Date object
-  // because user can mock it
-  var _Date = Date
   var startTimeCurrentSpec = new _Date().getTime()
 
   function handleGlobalErrors (result) {