title: PATCH (partial=true) doesn't work in Django
body: <p>I have a PATCH endpoint to change some non required data for a "Site". Through the endpoint you should be able to edit the description and supplier from a Site. The description is already working on the existing project. When I try to add the supplier to the PATCH, it doesn't update it..</p>
  
  <p>View:</p>
  
  <pre><code>class AdminSiteDetailView(GenericAPIView):
      def get_permissions(self):
          return IsAuthenticated(),
  
      def get_serializer_class(self):
          return AdminSiteDetailSerializer
  
      @swagger_auto_schema(
          responses={
              200: openapi.Response(
                  _("Successfully"),
                  AdminSiteDetailSerializer,
              )
          }
      )
      def get(self, request, site_pk):
          """
          GET the data from the site.
          """
          #somecode
  
      @swagger_auto_schema(
          responses={
              200: openapi.Response(
                  _("Successfully"),
                  AdminSiteDetailSerializer,
              )
          }
      )
      def patch(self, request, site_pk):
          """
          PATCH the description of the site.
          """
  
          site = get_object_or_404(Site, pk=site_pk)
          serializer_class = self.get_serializer_class()
          serializer = serializer_class(site, data=request.data, partial=True)
          serializer.is_valid(raise_exception=True)
          serializer.save()
          site.save()
          return Response(serializer.data, status=status.HTTP_200_OK)
  </code></pre>
  
  <p>Serializer:</p>
  
  <pre><code>class AdminSiteSerializer(serializers.ModelSerializer):
      supplier = serializers.SerializerMethodField()
  
      class Meta:
          model = Site
          fields = [
              "id",
              "name",
              "supplier",
          ]
  
      @staticmethod
      def get_supplier(site):
          if not site.supplier:
              return None
          return SupplierSerializer(site.supplier).data
  
  
  class AdminSiteDetailSerializer(AdminSiteSerializer):
      class Meta(AdminSiteSerializer.Meta):
          fields = AdminSiteSerializer.Meta.fields + ["description"]
  
  
  class SupplierSerializer(serializers.ModelSerializer):
      class Meta:
          model = Supplier
          fields = ("id", "name")
  </code></pre>
  
  <p>model:</p>
  
  <pre><code>class Site(models.Model):
      class Meta:
          ordering = ("name",)
  
      name = models.CharField(max_length=250)
      description = models.TextField(blank=True, null=True)
      supplier = models.ForeignKey(
          Supplier, on_delete=SET_NULL, blank=True, null=True, related_name="sites"
      )
  </code></pre>