Don't exclude null values from dynamic properties

When creating a document with 'dynamic properties', if a null value
is used, then the property is excluded from the document. e.g.

    Document(dict(a=None, b=None)).to_json()

Is there a reason for this that I'm missing?
Merge branch 'master' of https://github.com/benoitc/couchdbkit into patch-1
Merge branch 'patch-1'