Fixing the could not convert string to float: when we pass a empty string.
Merge branch 'master' of github.com:luanfonceca/django-hstore-flattenfields
Merge branch 'master' of github.com:multmeio/django-hstore-flattenfields
Fix the problem with _dfields cache and inheritance.
Merge pull request #14 from luanfonceca/master

Fixing the "could not convert string to float:"