Unable to add_machine with 'tags' as constraints

Calling add_machine with 'tags' as constraints fails as follows:

>>> from jujuclient.juju2 import environment
>>> juju = environment.Environment.connect("maas:default")
>>> juju.add_machine(series="xenial", constraints={'tags': 'test'})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/jujuclient/environment.py", line 473, in add_machine
    return self.client.add_machine(*args, **kws)
  File "/usr/local/lib/python2.7/dist-packages/jujuclient/juju2/facades.py", line 90, in add_machine
    return self.add_machines([params])['machines'][0]
  File "/usr/local/lib/python2.7/dist-packages/jujuclient/juju2/facades.py", line 100, in add_machines
    "params": machines}})
  File "/usr/local/lib/python2.7/dist-packages/jujuclient/facades.py", line 72, in rpc
    return self.env._rpc(self.check_op(op))
  File "/usr/local/lib/python2.7/dist-packages/jujuclient/rpc.py", line 42, in _rpc
    raise EnvError(result)
jujuclient.exc.EnvError: <Env Error - Details:
 { u'error': u'json: cannot unmarshal string into Go value of type []string',
    u'request-id': 2,
    u'response': { }}

This is because of the fact that the current implementation of the _prepare_constraints looks as follows:

 53 def _prepare_constraints(self, constraints):
 54 for k in ['cpu-cores', 'cpu-power', 'mem']:
 55 if constraints.get(k):
 56 constraints[k] = int(constraints[k])
 57 return constraints

On providers such as MAAS, I think it is normal that constraints such as 'tags' to be supported.