Fix access token audit tests for rails 5

Failure:
AccessTokenTest#test_creation_is_audited [/home/marta/DevRepos/3scale/porta/test/models/access_token_test.rb:142]
Minitest::Assertion: --- expected
+++ actual
@@ -1 +1 @@
-{"owner_id"=>12, "scopes"=>["stats"], "name"=>"Alaska_6", "permission"=>"rw", "created_at"=>2020-01-28 18:10:25 UTC, "updated_at"=>2020-01-28 18:10:25 UTC}
+{"owner_id"=>12, "owner_type"=>"User", "scopes"=>["stats"], "name"=>"Alaska_6", "permission"=>"rw", "created_at"=>Tue, 28 Jan 2020 18:10:25 UTC +00:00, "updated_at"=>Tue, 28 Jan 2020 18:10:25 UTC +00:00}


Failure:
AccessTokenTest#test_deletion_is_audited [/home/marta/DevRepos/3scale/porta/test/models/access_token_test.rb:158]
Minitest::Assertion: --- expected
+++ actual
@@ -1 +1 @@
-{"owner_id"=>13, "scopes"=>["stats"], "name"=>"Alaska_8", "permission"=>"rw", "created_at"=>2020-01-28 18:10:25 UTC, "updated_at"=>2020-01-28 18:10:25 UTC}
+{"owner_id"=>13, "owner_type"=>"User", "scopes"=>["stats"], "name"=>"Alaska_8", "permission"=>"rw", "created_at"=>Tue, 28 Jan 2020 18:10:25 UTC +00:00, "updated_at"=>Tue, 28 Jan 2020 18:10:25 UTC +00:00}


Failure:
AccessTokenTest#test_timestamps_filled [/home/marta/DevRepos/3scale/porta/test/models/access_token_test.rb:126]
Minitest::Assertion: --- expected
+++ actual
@@ -1 +1 @@
-2019-08-28 18:10:26 UTC
+Wed, 28 Aug 2019 18:10:26 UTC +00:00


Failure:
AccessTokenTest#test_update_is_audited [/home/marta/DevRepos/3scale/porta/test/models/access_token_test.rb:184]
Minitest::Assertion: --- expected
+++ actual
@@ -1 +1 @@
-{"name"=>["initial-name", "updated-name"], "updated_at"=>[2020-01-28 18:10:26 UTC, 2020-01-28 18:10:26 UTC]}
+{"name"=>["initial-name", "updated-name"]}


15 tests, 80 assertions, 4 failures, 0 errors, 0 skips