Add support for many=True on create

[feelixe]

This adds support for initializing serializer with many=True. Works for create, not update.

The current exception when attempting many=True in the master branch is that initial_data is not set.
So basically, I've created a ListSerializer BaseNestedListSerializer. And assign this in BaseNestedModelSerializer in method many_init. The list serializer then iterates over self.get_initial() and sets initial_data on the child serializer. I've tried running this in my project and so far I've not experienced any problems.

I would love some help to review this and if there are some improvements that can be made and what steps need to be taken for a merge.

Example usage (Same as current usage, except if you want to use many=True, you have to instantiate the serializer with that kwarg):

serializers.py

from drf_writable_nested import WritableNestedModelSerializer

class MyModelSerializer(WritableNestedModelSerializer):
    owner = PersonSerializer() # Some nested serializer
    toys = ToySerializer(many=True) # Another nested serializer

    class Meta:
        model = MyModel
        fields = '__all__'

views.py

from rest_framework import viewsets
from .serializers import MyModelSerializer
from .models import MyModel

class MyModelViewset(viewsets.ModelViewSet):
    serializer_class = MyModelSerializer
    queryset = MyModel.objects.all()

    def get_serializer(self, *args, **kwargs):
        if self.action == 'create':
            kwargs['many'] = True
        return super().get_serializer(*args, **kwargs)

Or to allow for sending one or many:

from rest_framework import viewsets
from .serializers import MyModelSerializer
from .models import MyModel

class MyModelViewset(viewsets.ModelViewSet):
    serializer_class = MyModelSerializer
    queryset = MyModel.objects.all()

    def get_serializer(self, *args, **kwargs):
        if self.action == 'create' and isinstance(kwargs['data'], list):
            kwargs['many'] = True
        return super().get_serializer(*args, **kwargs)

[codecov-commenter]

Codecov Report

Merging #151 (9096e78) into master (f7d470f) will decrease coverage by 1.98%.
The diff coverage is 52.94%.

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
- Coverage   98.16%   96.18%   -1.99%     
==========================================
  Files           3        3              
  Lines         218      262      +44     
==========================================
+ Hits          214      252      +38     
- Misses          4       10       +6     

Impacted Files	Coverage Δ
drf_writable_nested/mixins.py	96.00% <52.94%> (-2.06%)	⬇️

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[robd003]

Any chance @ruscoder or @ir4y could look at this?

[wael-kad]

Does the self.get_initial() in the create of BaseNestedListSerializer work if our main serializer has a PrimaryKeyRelatedField through a Foreign Key? 🤔

[auvipy]

why not also add the code examples as tests to verify the changes?