Fix for playlist.add_tracks! if no credentials for the playlist owner exist

[shaicoleman]

Calling add_tracks! causes the following exception:

/home/shai/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/rspotify-2.12.1/lib/rspotify/user.rb:65:in `oauth_header': undefined method `[]' for nil (NoMethodError)

        'Authorization' => "Bearer #{@@users_credentials[user_id]['token']}",
                                                                 ^^^^^^^^^
    from /home/shai/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/rspotify-2.12.1/lib/rspotify/user.rb:73:in `oauth_send'
    from /home/shai/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/rspotify-2.12.1/lib/rspotify/user.rb:87:in `block (2 levels) in <class:User>'
    from /home/shai/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/rspotify-2.12.1/lib/rspotify/playlist.rb:165:in `add_tracks!'    

This is because the add_tracks! tries to use the credentials of the playlist owner, instead of the current user's credentials. https://github.com/guilhermesad/rspotify/blob/0a95a52e1b2bbfbed65ff4b8cffee12beec74623/lib/rspotify/playlist.rb#L165

If the credentials for the users are not found, it will fall back to the first token.

Properly fixing it requires a breaking change, as you need to pass the user to the playlist, but this will fix it for most cases.

[guilhermesad]

Looks good, thanks!