Add a method to check if a key exists

[lesichkovm]

The current way to check if a key exists is to use Get and check for nil. While it works it create boilerplate code.

It would be most convenient if there was a method to check if a key exists. For instance:

if cache.Has("key123") {
    return cache.Get("key123")
}

Thanks

[swithek]

That's a good idea :+1: