Extended test with case of saveing model with blank not null field.
Charfied from_native method returns default instead of None. Updated tests.
In case of None value returns empty string instead of NoneType.
Fixed test.
Merge branch 'master' into modelserialization-charfield-with-null
Merge pull request #1665 from smal/modelserialization-charfield-with-null

Modelserialization charfield with null