allow https URLs

At the moment URLs in href tags are considered invalid. The RFC linked in the help text allows them (http://www.ietf.org/rfc/rfc3986.txt section 3.1).

Example to verify the error:

<itunes:image href="https://www.example.com/image.png"/>

The code change only makes the regex responsible for this accept http and https URLs. I also added a testcase for it. It did not break any of the other tests as far as I can see.

If you need anything else let me know.