Allow stub or spy Date object safely using a closure to get a clean copy

[tcorral]

Hi, I'm using Jasmine for my open source projects and in my job, and I had some problems when trying to stub or spy Date object using Jasmine spies or Sinon.js stubs.
The problem is that Jasmine uses the original Date object for timers and if you stub or spy the object then you can't use Jasmine because it fails when it tries to execute the 'getTime' method.
The solution I propose is to store a clean copy of the original Date object using a closure to store and use it even if the original object has been stubbed or spied in any other test.
I've not created a new test because the original behaviour should became the same.

[infews]

Good find. I'll take a deeper look at this shortly.

[sheelc]

Changed the implementation slightly since we already had a sufficient place to make the closure but should be merged. Thanks for the PR!