Add Attribute#defined? to return if attribute value has been defined
Use Attribute#default? in AttributeSet#skip_default?
Don’t override already-defined default values when freezing

Further, make `AttributeSet#set_default_attributes!` behavior respect
its documentation, which implied its only difference from
`set_default_attributes` was that it additionally set lazy attributes,
which was not entirely true since it also set non-lazy (and lazy)
attributes that had already been defined.
Merge pull request #240 from amarshall/freezing-defaults

Don’t override already-defined default values when freezing