`has_secure_password` is not invalid when assigning empty Strings.

Closes #9535.

With 692b3b6 the `password=` setter does no longer set blank passwords.
This triggered validation errors when assigning empty Strings to `password`
and `password_confirmation`.

This patch only sets the confirmation if it is not `blank?`.

activemodel/CHANGELOG.md

@@ -1,5 +1,15 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   `has_secure_password` does not fail the confirmation validation
+    when assigning empty String to `password` and `password_confirmation`.
+
+    Example:
+
+        # given User has_secure_password.
+        @user.password = ""
+        @user.password_confirmation = ""
+        @user.valid?(:update) # used to be false
+
 *   `validates_confirmation_of` does not override writer methods for
     the confirmation attribute if no reader is defined.
 

activemodel/lib/active_model/secure_password.rb

@@ -48,6 +48,8 @@ module ActiveModel
 
         attr_reader :password
 
+        include InstanceMethodsOnActivation
+
         if options.fetch(:validations, true)
           validates_confirmation_of :password
           validates_presence_of     :password, :on => :create
@@ -55,8 +57,6 @@ module ActiveModel
           before_create { raise "Password digest missing on new record" if password_digest.blank? }
         end
 
-        include InstanceMethodsOnActivation
-
         if respond_to?(:attributes_protected_by_default)
           def self.attributes_protected_by_default #:nodoc:
             super + ['password_digest']
@@ -99,6 +99,12 @@ module ActiveModel
           self.password_digest = BCrypt::Password.create(unencrypted_password, cost: cost)
         end
       end
+
+      def password_confirmation=(unencrypted_password)
+        unless unencrypted_password.blank?
+          @password_confirmation = unencrypted_password
+        end
+      end
     end
   end
 end

activemodel/test/cases/secure_password_test.rb

@@ -88,4 +88,10 @@ class SecurePasswordTest < ActiveModel::TestCase
     @user.password = "secret"
     assert_equal BCrypt::Engine::MIN_COST, @user.password_digest.cost
   end
+
+  test "blank password_confirmation does not result in a confirmation error" do
+    @user.password = ""
+    @user.password_confirmation = ""
+    assert @user.valid?(:update), "user should be valid"
+  end
 end