Use Object.keys task.config instead of undefined.

Fix for #4

To determine the default target for multi task, look at task.config
(if it was supplied).
Test that the task is run with the correct target
Merge remote-tracking branch 'origin/gh-4-broken-default'

* origin/gh-4-broken-default:
  Test that the task is run with the correct target
  Use Object.keys task.config instead of undefined.