Allow configuring the cache with a cache store instance

[pioz]

Problem

Geocoder::Cache picks its cache store adapter from the class name of the configured cache: object:

@class = (Geocoder::CacheStore.const_get("#{store.class}", false) rescue Geocoder::CacheStore::Generic)

Any object whose class is not literally named Redis silently falls back to Geocoder::CacheStore::Generic, which does not apply cache_options[:expiration]. In our app the Redis client is wrapped in a small namespacing class, so the configured expiration: 2.days was silently ignored and keys accumulated in Redis with no TTL — we only noticed when Redis memory kept growing. The same pitfall is described in #1670 (comment).

Passing an already-built cache store instance — as the README suggests for Rails (cache: Geocoder::CacheStore::Generic.new(Rails.cache, {})) — only works by accident today: Geocoder::Cache wraps the instance in a second Generic adapter, whose duck-typing dispatch happens to delegate write/read to the inner adapter. expire is actually broken in that setup, because Generic#remove calls store.delete, which cache store adapters don't implement:

NoMethodError: undefined method 'delete' for an instance of Geocoder::CacheStore::Generic

Change

When the configured cache: is already a Geocoder::CacheStore::Base instance, use it directly instead of wrapping it. This gives an explicit way to choose the adapter for stores whose class name is not recognized:

Geocoder.configure(
  cache: Geocoder::CacheStore::Redis.new(MyRedisWrapper.new, {prefix: "geocoder:", expiration: 2.days})
)

Expiration support is still never guessed for unrecognized stores (in line with the intent expressed in #1670): the user opts in explicitly by picking the adapter.

Backward compatible: the class name lookup and the Generic fallback are unchanged for every other value of cache:. The README example for Rails.cache now also works fully (no double wrapping, expire included).

Related to #1670.

[alexreisner]

This is excellent. Thank you for the great code and clear explanation. Merging!

[pioz]

Yeah! Any change to get a new geocoder release (1.8.7) with this update?