Allow storing static preferences using string class names

This will make unnecessary to wrap these definitions under `.to_prepare`
blocks.

Additionally the error message for unexpected keys has improved and
now lists both all the unexpected keys along with the expected keys,
mentioning the name associated to the definition.

The only thing that changed is that it doesn't do so when the
configuration is added but rather when it's fetched.

core/lib/spree/preferences/static_model_preferences.rb

@@ -7,13 +7,8 @@ class Definition
         attr_reader :preferences
 
         def initialize(klass, hash)
-          hash = hash.symbolize_keys
-          hash.keys.each do |key|
-            if !klass.defined_preferences.include?(key)
-              raise "Preference #{key.inspect} is not defined on #{klass}"
-            end
-          end
-          @preferences = hash
+          @klass = klass
+          @preferences = hash.symbolize_keys
         end
 
         def fetch(key, &block)
@@ -27,20 +22,29 @@ def []=(key, value)
         def to_hash
           @preferences.deep_dup
         end
+
+        delegate :keys, to: :@preferences
       end
 
       def initialize
-        @store = Hash.new do |data, klass|
-          data[klass] = {}
-        end
+        @store = {}
       end
 
       def add(klass, name, preferences)
-        @store[klass.to_s][name] = Definition.new(klass, preferences)
+        @store[klass.to_s] ||= {}
+        @store[klass.to_s][name] = Definition.new(klass.to_s, preferences)
       end
 
       def for_class(klass)
-        @store[klass.to_s]
+        (@store[klass.to_s] || {}).tap do |names_with_prefs|
+          names_with_prefs.each do |name, preferences|
+            klass_keys = klass.defined_preferences.map(&:to_s)
+            extra_keys = preferences.keys.map(&:to_s) - klass_keys
+            next if extra_keys.empty?
+
+            raise "Unexpected keys found for #{klass} under #{name}: #{extra_keys.to_sentence} (expected keys: #{klass_keys.to_sentence})"
+          end
+        end
       end
     end
   end

core/spec/models/spree/preferences/static_model_preferences_spec.rb

@@ -32,9 +32,13 @@ module Spree
     end
 
     it "errors assigning invalid preferences" do
+      subject.add(preference_class, 'my_definition', { ice_cream: 'chocolate' })
+
       expect {
-        subject.add(preference_class, 'my_definition', { ice_cream: 'chocolate' })
-      }.to raise_error(/\APreference :ice_cream is not defined/)
+        subject.for_class(preference_class)
+      }.to raise_error(
+        /\AUnexpected keys found for #{Regexp.escape preference_class.to_s} under my_definition: ice_cream \(expected keys: color\)/
+      )
     end
 
     context "with stored definitions" do