Ruby/Rugged How can I save change after a git fetch?

Question

So, I wanted to perform a git pull with rugged, so I make a fetch and merge like this:

require 'rugged'

 certificat = Rugged::Credentials::SshKey.new({username: 'git', privatekey: 'path/to/privatekey', publickey: 'path/to/publickey' })

  repo = Rugged::Repository.new("/tmp/git")
  repo.checkout('master')

  # git fetch

  remote = repo.remotes['origin']
  remote.fetch(credentials: certificat)
  remote.save # save new data

  # git merge

  commit = repo.branches['origin/master'].target
  repo.references.update(repo.head, commit.oid)

But I have this error with the save method:

undefined method `save' for #<Rugged::Remote:0x0000000135d6e8> (NoMethodError)

I don't understand why especially that the save method is in the Rugged doc (here)

Somebody know why ?

EDIT: Ok, so this documentation is outdated, the method save doesn't exist anymore. I think my merge is imcomplete, somebody know ?

EDIT2: I just add this line at the end of this code and it's work !

repo.checkout_head({strategy: :force})