Fix the Memcached::Rails fetch method to work when no block is given.

Fix Memcached::Rails fetch method to behave like ActiveSupport's fetch method when no block is given.  The fetch method currently returns an error if no block is given.  This is differs from the fetch behavior from ActiveSupport caches.  According to the documentation for fetch:

```ruby
    # Fetches data from the cache, using the given key. If there is data in
    # the cache with the given key, then that data is returned.
    #
    # If there is no such data in the cache (a cache miss), then nil will be
    # returned. However, if a block has been passed, that block will be run
    # in the event of a cache miss. The return value of the block will be
    # written to the cache under the given cache key, and that return value
    # will be returned.
```