Warning about "Multi" when using a single buildpack looks confusing

[jarthod]

When replacing the default buildpack with a custom one using the buildpacks:add command, we get the following Warning:

-----> Warning: Multiple default buildpacks reported the ability to handle this app. The first buildpack in the list below will be used.
       Detected buildpacks: multi ruby
-----> Multipack app detected

Which has no reason to be here because there's only one buildpack so there's no ambiguity, this warning make it looks like we did something wrong and we're not sure if it's gonna execute two buildpacks. Apparently it doesn't but in this case the warning is misleading.

Reproduce (dokku 0.20.2):

> dokku buildpacks:add https://github.com/jarthod/heroku-buildpack-ruby.git
> dokku buildpacks:list
-----> suzaku buildpack urls
       https://github.com/jarthod/heroku-buildpack-ruby.git
> dokku ps:rebuild
-----> Cleaning up...
-----> Building suzaku from herokuish...
-----> Adding BUILD_ENV to build environment...
-----> Warning: Multiple default buildpacks reported the ability to handle this app. The first buildpack in the list below will be used.
       Detected buildpacks: multi ruby
-----> Multipack app detected
=====> Downloading Buildpack: https://github.com/jarthod/heroku-buildpack-ruby.git
=====> Detected Framework: Ruby
...