add test for None values
skip None values in get_prep_value()
Merge pull request #15 from fdemmer/nonevalues

Support NULL values
Add failing test for #11
Respect Django DeserializedObject
Fix deserialization issue
Make test fail again
Serialize DictionaryField using JSON
Fix python3 compatibility introduced in last commits.
Merge branch 'boronine-master'
add tests for ModelForm with DictionaryField
use initial in form field instead of model field
make DictionaryField (model field) deliver a string value
update DictionaryFieldWidget render function for field value change
add key sorting value_from_datadict()
provide ValidationErrors for form input fields
Fix compatibility with previos pull-request changes.
Merge branch 'fdemmer-forms'