Add `Devise.activerecord51?`

Active Record changed it's public API, so we should check against its
version instead of Rails as it is possible to use Rails 5.1 with Mongoid,
which still has the older Dirty API.

However, this patch does not fixes a scenario where an app has both
Active Record and Mongoid loaded. It should be fixed by either normalizing
the Mongoid/ActiveRecord API or replacing the conditional method
definitions with a shim layer that abstracts this away.

lib/devise.rb

@@ -294,6 +294,10 @@ module Devise
     Rails.gem_version >= Gem::Version.new("5.1.x")
   end
 
+  def self.activerecord51? # :nodoc:
+    defined?(ActiveRecord) && ActiveRecord.gem_version >= Gem::Version.new("5.1.x")
+  end
+
   # Default way to set up Devise. Run rails generate devise_install to create
   # a fresh initializer with all configuration values.
   def self.setup

lib/devise/models/confirmable.rb

@@ -253,7 +253,7 @@ module Devise
           generate_confirmation_token && save(validate: false)
         end
 
-        if Devise.rails51?
+        if Devise.activerecord51?
           def postpone_email_change_until_confirmation_and_regenerate_confirmation_token
             @reconfirmation_required = true
             self.unconfirmed_email = self.email
@@ -271,7 +271,7 @@ module Devise
           end
         end
 
-        if Devise.rails51?
+        if Devise.activerecord51?
           def postpone_email_change?
             postpone = self.class.reconfirmable &&
               will_save_change_to_email? &&

lib/devise/models/database_authenticatable.rb

@@ -137,7 +137,7 @@ module Devise
         encrypted_password[0,29] if encrypted_password
       end
 
-      if Devise.rails51?
+      if Devise.activerecord51?
         # Send notification to user when email changes.
         def send_email_changed_notification
           send_devise_notification(:email_changed, to: email_before_last_save)
@@ -165,7 +165,7 @@ module Devise
         Devise::Encryptor.digest(self.class, password)
       end
 
-      if Devise.rails51?
+      if Devise.activerecord51?
         def send_email_changed_notification?
           self.class.send_email_changed_notification && saved_change_to_email?
         end
@@ -175,7 +175,7 @@ module Devise
         end
       end
 
-      if Devise.rails51?
+      if Devise.activerecord51?
         def send_password_change_notification?
           self.class.send_password_change_notification && saved_change_to_encrypted_password?
         end

lib/devise/models/recoverable.rb

@@ -97,7 +97,7 @@ module Devise
           send_devise_notification(:reset_password_instructions, token, {})
         end
 
-        if Devise.rails51?
+        if Devise.activerecord51?
           def clear_reset_password_token?
             encrypted_password_changed = respond_to?(:will_save_change_to_encrypted_password?) && will_save_change_to_encrypted_password?
             authentication_keys_changed = self.class.authentication_keys.any? do |attribute|

lib/devise/models/validatable.rb

@@ -27,7 +27,7 @@ module Devise
 
         base.class_eval do
           validates_presence_of   :email, if: :email_required?
-          if Devise.rails51?
+          if Devise.activerecord51?
             validates_uniqueness_of :email, allow_blank: true, if: :will_save_change_to_email?
             validates_format_of     :email, with: email_regexp, allow_blank: true, if: :will_save_change_to_email?
           else