Cannot set user for juju scp

The scp command uses a user@hostname format to allow the user to be specified, as does ssh.

The juju versions of these commands do not accept this format:

$ juju scp buildvars.json jenkins@jenkins/0:
ERROR exit status 1 (cp: cannot create regular file ‘ubuntu@jennkins@jenkins/0:’: No such file or directory)

With ssh, the -l parameter can be used instead, but scp does not accept -l. scp accepts -o, but only before the file list, and "juju scp" interprets -o before the file list as a "juju scp" option, not an "scp" option:

$ juju scp -o User=jenkins buildvars.json jenkins/0:
error: flag provided but not defined: -o

This invocation is accepted:
$ juju scp -- -o User=jenkins buildvars.json jenkins/0:

But the user is still set to "ubuntu", even though normal scp honours "-o User=jenkins". It is also counter-intuitive, because -- should mean that juju passes "jenkins/0:" to scp verbatim.