Fixes #371 - Support ListSerializer fields in SerializerMutation

[mbarrientos]

This allows SerializerMutation to handle serializer fields with many=True.

Fixes #371

e.g. having these serializers

class CategorySerializer(ModelSerializer):
    name = serializers.CharField()

    class Meta:
        model = Category
        fields = '__all__'


class ProductSerializer(ModelSerializer):
    description = serializers.CharField()
    category = CategorySerializer(many=True)

    class Meta:
        model = Product
        fields = '__all__'

... and this mutation

class CreateProduct(SerializerMutation):
    class Meta:
        serializer_class = ProductSerializer

enables nested objects in in the queries:

mutation {
	createProduct(input:{
    description: "Product description",
    category: [
      {name: "Category 1"}
    ]
  }) {
    id
  }
}

[coveralls]

Coverage decreased (-0.3%) to 94.237% when pulling eacdb50 on mbarrientos:drf-list-serializers into dbd3957 on graphql-python:master.

[coveralls]

Coverage decreased (-0.3%) to 94.237% when pulling eacdb50 on mbarrientos:drf-list-serializers into dbd3957 on graphql-python:master.

[coveralls]

Coverage decreased (-0.3%) to 94.237% when pulling eacdb50 on mbarrientos:drf-list-serializers into dbd3957 on graphql-python:master.

[coveralls]

Coverage decreased (-0.3%) to 94.237% when pulling eacdb50 on mbarrientos:drf-list-serializers into dbd3957 on graphql-python:master.

[coveralls]

Coverage increased (+0.06%) to 94.641% when pulling 3793ce1 on mbarrientos:drf-list-serializers into dbd3957 on graphql-python:master.

[syrusakbary]

Gracias por el PR Miguel!

Merging :)