Expose mailbox_for method

Currently, the only exposed entry point into the ApplicationMailbox's configured
routing system is to call `route`, which performs a lot of work to fully
`process` inbound email. It'd be nice to have a way (e.g. in test) of checking
which mailbox an email would route to without necessarily processing it yet.

actionmailbox/lib/action_mailbox/router.rb

@@ -21,7 +21,7 @@ module ActionMailbox
     end
 
     def route(inbound_email)
-      if mailbox = match_to_mailbox(inbound_email)
+      if mailbox = mailbox_for(inbound_email)
         mailbox.receive(inbound_email)
       else
         inbound_email.bounced!
@@ -30,12 +30,12 @@ module ActionMailbox
       end
     end
 
+    def mailbox_for(inbound_email)
+      routes.detect { |route| route.match?(inbound_email) }.try(:mailbox_class)
+    end
+
     private
       attr_reader :routes
-
-      def match_to_mailbox(inbound_email)
-        routes.detect { |route| route.match?(inbound_email) }.try(:mailbox_class)
-      end
   end
 end
 

actionmailbox/lib/action_mailbox/routing.rb

@@ -17,6 +17,10 @@ module ActionMailbox
       def route(inbound_email)
         router.route(inbound_email)
       end
+
+      def mailbox_for(inbound_email)
+        router.route(inbound_email)
+      end
     end
   end
 end

actionmailbox/test/unit/mailbox/routing_test.rb

@@ -28,4 +28,9 @@ class ActionMailbox::Base::RoutingTest < ActiveSupport::TestCase
       assert_equal "Discussion: Let's debate these attachments", $processed
     end
   end
+
+  test "mailbox_for" do
+    mail = create_inbound_email_from_fixture "welcome.eml", status: :pending
+    assert_equal RepliesMailbox, ApplicationMailbox.mailbox_for(mail)
+  end
 end

actionmailbox/test/unit/router_test.rb

@@ -135,5 +135,19 @@ module ActionMailbox
         @router.add_route Array.new, to: :first
       end
     end
+
+    test "single string mailbox_for" do
+      @router.add_routes("first@example.com" => :first)
+
+      inbound_email = create_inbound_email_from_mail(to: "first@example.com", subject: "This is a reply")
+      assert_equal FirstMailbox, @router.mailbox_for(inbound_email)
+    end
+
+    test "mailbox_for with no matches" do
+      @router.add_routes("first@example.com" => :first)
+
+      inbound_email = create_inbound_email_from_mail(to: "second@example.com", subject: "This is a reply")
+      assert_nil @router.mailbox_for(inbound_email)
+    end
   end
 end