Expose cache prefix as a constant

Now that `cache_valid_auth` is exposed as a helper it is possible to set
a value without duplicating the cache key logic. However, just to be
safe this exposes the prefix in a constant in case we need it later.

Author: cupakromer

lib/kracken/controllers/token_authenticatable.rb

@@ -31,8 +31,10 @@ def request_http_token_authentication(realm = 'Application')
 
     module_function
 
+      TOKEN_AUTH_CACHE_PREFIX = "auth/token/"
+
       def cache_valid_auth(token, &generate_cache)
-        cache_key = "auth/token/#{token}"
+        cache_key = TOKEN_AUTH_CACHE_PREFIX + token
         val = Rails.cache.read(cache_key)
         val ||= store_valid_auth(cache_key, &generate_cache)
         shallow_freeze(val)