[Html-widget] "name" attribute not set by HTML::Widget::Element::Span::containerize()

[Will Hawes]

As above - most (if not all) of the other ::Element::* classes set the
element name in their containerize() method. Span doesn't.

My guess is that it's an oversight, because it's often useful to be
able to access an element's name within a template.

It appears to be a single line fix, will a patch for this be accepted?