Fix argument error on Ruby3.0.preview1 and ActionMailer

Error was

+  1) InboxMailbox route email to properly mailbox
+     Failure/Error:
+       expect(InboxMailbox)
+         .to receive_inbound_email(to: "[email protected]")
+
+     ArgumentError:
+       wrong number of arguments (given 1, expected 0)
+     # /home/runner/work/rspec-rails/bundle/ruby/3.0.0/gems/actionmailbox-6.0.3.4/lib/action_mailbox/test_helper.rb:16:in `create_inbound_email_from_mail'
+     # ./spec/mailboxes/inbox_mailbox_spec.rb:6:in `block (2 levels) in <top (required)>'

It should be fixed with Rails 6.1 rails/rails#35904

Author: benoittgt

.github/workflows/ci.yml

@@ -18,7 +18,6 @@ jobs:
        include:
          # Rails 6.0 builds >= 2.5.0
          - ruby: 3.0.0-preview1
-           allow_failure: true
            env:
              RAILS_VERSION: '~> 6.0.0'
          - ruby: 2.7

lib/rspec/rails/example/mailbox_example_group.rb

@@ -13,7 +13,7 @@ module MailboxExampleGroup
         def self.create_inbound_email(arg)
           case arg
           when Hash
-            create_inbound_email_from_mail(arg)
+            create_inbound_email_from_mail(**arg)
           else
             create_inbound_email_from_source(arg.to_s)
           end