RSS items without enclosure tag throw error

[jeyemwey]

If we ask metalove to parse a RSS feed which comes without enclosure tag (e.g. because we chose the blog feed instead of the podcast feed), sweet_xml won't find an enclosure length (~x"enclosure/@length"i) to parse to int. Thus, sweet_xml crashes here.

I will send a pull request which will make the tag/attribute optional.