Allow customising ChoiceField blank display value
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.
Added missing import.
Updated Django versions.
Fixed a small change in the 1.7 beta url.
fix pep8
Merge pull request #1589 from the5fire/fix-pep8

fix pep8
Document ChoiceField blank_display_value parameter
Merge pull request #1536 from Ian-Foote/choicefield_blank_display_value

Allow customising ChoiceField blank display value
fields: allow help_text on SerializerMethodField

...by passing through any extra *args and **kwargs
to the parent constructor.

Previously one couldn't assign help_text to a
SerializerMethodField during construction.
Merge pull request #1594 from jcsp/wip-methodfield

fields: allow help_text on SerializerMethodField
Fix _resolve_model to work with unicode strings
Merge pull request #1596 from pipermerriam/piper/suport_unicode_model_references

`_resolve_model` should work with unicode strings
Add the allow_add_remove parameter to the get_serializer method
Merge pull request #1603 from khamaileon/master

Add the allow_add_remove parameter to the get_serializer method
Note on configuring TokenAuthentication
Always use specified content type in APIRequestFactory

If `content_type` is specified in the `APIRequestFactory`, always
include it in the request, even if data is empty.