Fix Add to Overwrite existing Cache item

[glav]

There is a different between Add and Set within System.Runtime.Caching

Add does nothing if the cache key already exists.

I suggest either creating a similar function for this library or change the behaviour of Add so that it overwrites the value all times.

Often you will want to update the value manually after its already loaded (after a action) and decaching causes the method to run the logic again duplicating the effort.

Note: Copied from old bitbucket repo