story add_label fails when there is no existing labels

[robinw777]

Try on a story with no existing labels

s = client.story('__id__')
s.labels    # => nil
s.add_label("foo")  # => TypeError: can't dup NilClass
from ...tracker_api-1.4.0/lib/tracker_api/resources/story.rb:77:in `dup'

Note if the story has existing label, add_label works fine