Attempt to coerce the generated_at cookie to a Time object.

Time objects aren't properly coerced back when using the JSON cookie serialization,
so we need to do it ourselves.

To avoid any new JSON serialization issues, we now store the `generated_at` as
an String with the timestamp seconds + miliseconds in the cookie but still the
previous JSON encoded format.

Thanks to @boblail at https://github.com/plataformatec/devise/pull/3917 for the
initial patch.

lib/devise/models/rememberable.rb

@@ -97,6 +97,12 @@ module Devise
       end
 
       def remember_me?(token, generated_at)
+        # TODO: Normalize the JSON type coercion along with the Timeoutable hook
+        # in a single place https://github.com/plataformatec/devise/blob/ffe9d6d406e79108cf32a2c6a1d0b3828849c40b/lib/devise/hooks/timeoutable.rb#L14-L18
+        if generated_at.is_a?(String)
+          generated_at = time_from_json(generated_at)
+        end
+
         # The token is only valid if:
         # 1. we have a date
         # 2. the current time does not pass the expiry period
@@ -109,10 +115,20 @@ module Devise
          Devise.secure_compare(rememberable_value, token)
       end
 
+      private
+
+      def time_from_json(value)
+        if value =~ /\A\d+\.\d+\Z/
+          Time.at(value.to_f)
+        else
+          Time.parse(value) rescue nil
+        end
+      end
+
       module ClassMethods
         # Create the cookie key using the record id and remember_token
         def serialize_into_cookie(record)
-          [record.to_key, record.rememberable_value, Time.now.utc]
+          [record.to_key, record.rememberable_value, Time.now.utc.to_f.to_s]
         end
 
         # Recreate the user based on the stored cookie

test/models/rememberable_test.rb

@@ -37,7 +37,7 @@ class RememberableTest < ActiveSupport::TestCase
     id, token, date = User.serialize_into_cookie(user)
     assert_equal id, user.to_key
     assert_equal token, user.authenticatable_salt
-    assert date.is_a?(Time)
+    assert date.is_a?(String)
   end
 
   test 'serialize from cookie' do
@@ -46,6 +46,12 @@ class RememberableTest < ActiveSupport::TestCase
     assert_equal user, User.serialize_from_cookie(user.to_key, user.authenticatable_salt, Time.now.utc)
   end
 
+  test 'serialize from cookie should accept a String with the datetime seconds and microseconds' do
+    user = create_user
+    user.remember_me!
+    assert_equal user, User.serialize_from_cookie(user.to_key, user.authenticatable_salt, Time.now.utc.to_f.to_json)
+  end
+
   test 'serialize from cookie should return nil with invalid datetime' do
     user = create_user
     user.remember_me!