Eagerly check for store validity

Ref: #315

`respond_to?` is a relatively expensive method that doesn't
benefit from inline caching, so it has to keep looking up
the method every time. Hence it's best to avoid it in hotspots.

In this case, `@store` can only change through the `store=` method,
so we might as well eagerly validate it there.

First because this way it's a one time fixed cost, but also because
this way the error is raised early during boot/configuration rather
than at runtime where it has availability impact.

Author: byroot

lib/rack/attack/cache.rb

@@ -26,6 +26,12 @@ def store=(store)
           else
             store
           end
+        if @store
+          enforce_store_method_presence!(:read)
+          enforce_store_method_presence!(:write)
+          enforce_store_method_presence!(:delete)
+          enforce_store_method_presence!(:increment)
+        end
       end
 
       def count(unprefixed_key, period)
@@ -34,13 +40,14 @@ def count(unprefixed_key, period)
       end
 
       def read(unprefixed_key)
-        enforce_store_presence!
-        enforce_store_method_presence!(:read)
+        raise Rack::Attack::MissingStoreError if store.nil?
 
         store.read("#{prefix}:#{unprefixed_key}")
       end
 
       def write(unprefixed_key, value, expires_in)
+        raise Rack::Attack::MissingStoreError if store.nil?
+
         store.write("#{prefix}:#{unprefixed_key}", value, expires_in: expires_in)
       end
 
@@ -74,26 +81,17 @@ def key_and_expiry(unprefixed_key, period)
       end
 
       def do_count(key, expires_in)
-        enforce_store_presence!
-        enforce_store_method_presence!(:increment)
+        raise Rack::Attack::MissingStoreError if store.nil?
 
         result = store.increment(key, 1, expires_in: expires_in)
 
         # NB: Some stores return nil when incrementing uninitialized values
         if result.nil?
-          enforce_store_method_presence!(:write)
-
           store.write(key, 1, expires_in: expires_in)
         end
         result || 1
       end
 
-      def enforce_store_presence!
-        if store.nil?
-          raise Rack::Attack::MissingStoreError
-        end
-      end
-
       def enforce_store_method_presence!(method_name)
         if !store.respond_to?(method_name)
           raise(