Handle tvshows where there is only one actor (Issue #229)

As per the issue here:
https://github.com/SiCKRAGETV/sickrage-issues/issues/229

SickRage metadata handling does not seem to handle shows with only one actor returned by the tvdb API.
It calls items() on a dict.

I'm not a python guy, but the below fixed worked locally, basically just putting the 'actor' dict into a list if it is not already one.