Make CAS play nice with Snappy

`cas` doesn't play nicely with `MemcachedSnappyStore` because `add` always sidesteps the `Memcached` codec, while `cas` honours the `:raw` option.

This adds tests for `cas` and `cas_multi` with `MemcachedSnappyStore`, with and without `:raw => true`.

It fixes the problem by adding a `cas_raw?` method to `MemcachedStore` that returns `options[:raw]`. `MemcachedSnappyStore` overrides `cas_raw?` to return true.

`cas_raw?` is used to compute the `raw` argument to `cas` and `cas_multi`, while `options` is passed unfettered to `serialize_entry` within the `cas` and `cas_multi` blocks.

@camilo @arthurnn for review. /cc @Shopify/webscale