amulet not respecting series= declaration for multi-series charms

[lazypower]

With the following snippet in a layers metadata.yaml

series:
  - trusty
  - xenial

And the following defined in a test in tests/10-deploy-test.py

        cls.deployment = amulet.Deployment(series='xenial')

I will get a trusty series charm every time i execute the tests. I've discovered that if you move 'xenial' to the top of the series list on the output charm, it will deploy the series of xenial, but it appears the series= directive was not respected.

ii amulet 1.14.4-0ubuntu4~ubuntu1 all Testing harness for Juju Charms