Update docs, error msg for cache store #keys (#1165)

README.md

@@ -896,6 +896,7 @@ This example uses Redis, but the cache store can be any object that supports the
 * `store#[](key)` or `#get` or `#read` - retrieves a value
 * `store#[]=(key, value)` or `#set` or `#write` - stores a value
 * `store#del(url)` - deletes a value
+* `store#keys` - (Optional) Returns array of keys. Used if you wish to expire the entire cache (see below).
 
 Even a plain Ruby hash will work, though it's not a great choice (cleared out when app is restarted, not shared between app instances, etc).
 

lib/geocoder/cache.rb

@@ -40,7 +40,11 @@ module Geocoder
     #
     def expire(url)
       if url == :all
-        urls.each{ |u| expire(u) }
+        if store.respond_to?(:keys)
+          urls.each{ |u| expire(u) }
+        else
+          raise(NoMethodError, "The Geocoder cache store must implement `#keys` for `expire(:all)` to work")
+        end
       else
         expire_single_url(url)
       end