Bugfix for parsing rounded rect instances

[kennyweiss]

This PR contains a minor bugfix for parsing rounded rectangles.

When calling rect2pathd through the Document class, the passed in Element instance did not directly expose its rx and ry attributes, which can be accessed through the keys() method.

I added unit tests for this case and verified that everything continues to work when calling rect2pathd with a dictionary and with the svg2paths function on an SVG document.