image: store href attr as a String
image: store href attr as a String

Parsing it as a HRef is not very useful since Href::WithFragment is not
a valid value anyway.
The url is checked once the image is loaded and if it is not valid, we
will get a io error.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg