Implement $cacheFactory.Cache#has method

[caitp]

When checking in #8214 I was honestly surprised we don't expose a way to check if a key is in the cache.

It would be helpful to have a simple implementation, like has: function(key) { return key in data; } or something.

@IgorMinar this is a public API change so I didn't just write a patch, but what do you think? It's pretty trivial and better than testing if the value of a key is undefined

[btford]

I like it

[Narretz]

See #14035 (comment)