Issue 4: sparse declarative defaults

[TadLeonard]

Declarative defaults now handle sparseness

A fix for #4. The old way of setting SparseEnumap defaults (SparseEnumap.set_defaults(...)) handled sparseness correctly. That is, calling set_defaults(*args, **kwargs) created a defaults dictionary with None placeholders. For example:

class People(SparseEnumap):
    donald = auto()
    vladimir = auto()
    william = auto

People.set_defaults(donald=1, william=5)
People.defaults()   # {"donald": 1, "vladimir": None, "william": 5}

The declarative version:

class People(SparseEnumap):
    donald = auto()
    vladimir = auto()
    william = auto

People.defaults()   # {"donald": 1, "william": 5}

... was broken. Each member in a SparseEnumap should have a default no matter what.

[jonemo]

lgtm

[TadLeonard]

Thanks, @jonemo. Merging so that tests with sparse types for PR #3 can be written.