Allow prevention of caching "null" strings/objects

[mrnohr]

Currently, if I use the redisService.memoize() or @memoize annotation, it will store the result in redis even if the value is null. There are cases where I do not want to cache null values.

For example, if I'm using @MemoizeDomainObject on a find method like this:

@MemoizeDomainObject(key = "#{title}", clazz = Book.class)
def findBookByTitle(String title){
    def book = // some expensive find operation
    return book
}

I only want to cache this if a book is found. If not found (and null is returned), then I always want to look up the value again in case the book was added. I am actually more interested in the @MemoizeObject annotation, but it would apply to @MemoizeDomainObject as well.

I was thinking some kind of flag on the annotation.

For now when I want this behavior, I use the annotation(s) and then delete the redis key if null is returned. This is 3 redis calls (GET to check the key, SETEX to add null, then DEL to remove it) when I'd rather have 1 redis call (GET) in this case.

[tednaleid]

I added this to the new memoizeObject method (that I implemented for #34). It's not currently on all of the annotations as I'm not sure quite how much this will get used, but could see some use-cases for it and it was easy enough to put on the new method.