Fall back to fillDefault when fill is not supplied

fillDefault is defined in
http://www.w3.org/TR/SMIL2/smil-timing.html#adef-fillDefault

When we read the element attributes, 'fillDefault' is exposed by Chrome
as 'filldefault' (no caps)

When fill is 'default' or not supplied, we fall back to using the
(possibly inherited) fillDefault value.
We should also test with dur unspecified to test the fill -> auto case.