Validate presence of provider field in Identity

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/identity.rb

@@ -14,6 +14,7 @@ class Identity < ActiveRecord::Base
   include Sortable
   belongs_to :user
 
+  validates :provider, presence: true
   validates :extern_uid, allow_blank: true, uniqueness: { scope: :provider }
   validates :user_id, uniqueness: { scope: :provider }
 end