Minor changes on avatar cropping internals

- Avoid multiple calls to `validates` for the avatar attributes.
- In a cropping process, don't check if the model inherits `User`, check if it responds to `:avatar_crop_size`.

app/models/user.rb

@@ -166,9 +166,10 @@ class User < ActiveRecord::Base
   validate :owns_public_email, if: ->(user) { user.public_email_changed? }
   validates :avatar, file_size: { maximum: 200.kilobytes.to_i }
 
-  [:avatar_crop_x, :avatar_crop_y, :avatar_crop_size].each do |field|
-    validates field, numericality: { only_integer: true }, presence: true, if: ->(user) { user.avatar_changed? }
-  end
+  validates :avatar_crop_x, :avatar_crop_y, :avatar_crop_size,
+    numericality: { only_integer: true },
+    presence: true,
+    if: ->(user) { user.avatar_changed? }
 
   before_validation :generate_password, on: :create
   before_validation :restricted_signup_domains, on: :create

app/uploaders/avatar_uploader.rb

@@ -11,12 +11,12 @@ class AvatarUploader < CarrierWave::Uploader::Base
   process :cropper
 
   def cropper
-    if model.kind_of?(User) && model.valid?
+    return unless model.respond_to?(:avatar_crop_size) && model.valid?
+
     manipulate! do |img|
       img.crop "#{model.avatar_crop_size}x#{model.avatar_crop_size}+#{model.avatar_crop_x}+#{model.avatar_crop_y}"
     end
   end
-  end
 
   def store_dir
     "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"