Regression test for #532 - values overriden by "default" setting
Fix "default" and partial serializers, fixes #532
Add explaining comment
Add `.patch()` method for `RetrieveUpdateAPIView`
Regression test for #542
FileField validation - handle case when files=None, fixes #542
Some cleanup
Updated release-notes
Merge pull request #545 from yprez/filefield-no-files_issue542

FileField validation with files=None
Move relation tests models into models.py
Merge pull request #546 from maspwr/refactor-relation-models

Move relation tests models into models.py
Fix copyright notice and travis badge
Drop unused private save_m2m flag
Tweak behavior of hyperlinked fields that include an explicit format suffix.
Tweak behavior of hyperlinked fields with an explicit format suffix.
Merge pull request #541 from yprez/default_and_partial_serializers2

Fix "default" values and partial serializers.  Fixes #532.
Tweak comment.
Tweak release notes.
Version 2.1.15
Remove duplicate release notes line