Types for fromUrl, parseHtml are too strict

[peter50216]

Current type for fromUrl and parseHtml are both:

String -> IOSArrow XmlTree (NTree XNode)

But it only needs to be

String -> IOSArrow b (NTree XNode)

This change would make the following code valid:

let doc = setDefaultBaseURI url >>> parseHtml html

[egonSchiele]

Pushed a fix, please try it out and see if it works.