Changes gradientUnits default value to objectBoundingBox

[fabionuno]

According to the W3C recommendation the default value of gradientUnits attribute is objectBoundingBox if not specified.

This PR changes SVG render to use objectBoundingBox as default value instead userSpaceOnUse.

[jonobr1]

I appreciate you trying to add parity with the W3C.., however I've found that userSpaceOnUse is actually more useful and intuitive than objectBoudningBox which is why I changed it. Do you have any examples using objectBoundingBox instead of userSpaceOnUser? It would help clarify justification. Thanks!