Bug in serializer when passing params

[Kandeel4411]

Hi!
Thanks for the great library! I faced an issue with multiple nested fields expansion and was able to track it down to the following lines:

drf-flex-fields/rest_flex_fields/serializers.py

Lines 93 to 107 in 25be9c1

	if name in nested_expand:
	settings["expand"] = nested_expand[name]
	if name in nested_fields:
	settings["fields"] = nested_fields[name]
	if name in nested_omit:
	settings["omit"] = nested_omit[name]
	if type(serializer_class) == str:
	serializer_class = self._get_serializer_class_from_lazy_string(
	serializer_class
	)
	return serializer_class(**settings)

TypeError: __init__() got an unexpected keyword argument 'expand'

Since I am using custom REST_FLEX_FIELDS params, this is throwing an error because the serializer expected parameters with different names since you only pop off the expected custom params

drf-flex-fields/rest_flex_fields/serializers.py

Lines 26 to 31 in 25be9c1

	def __init__(self, *args, **kwargs):
	expand = list(kwargs.pop(EXPAND_PARAM, []))
	fields = list(kwargs.pop(FIELDS_PARAM, []))
	omit = list(kwargs.pop(OMIT_PARAM, []))
	super(FlexFieldsSerializerMixin, self).__init__(*args, **kwargs)

I believe all thats needed to fix this is to edit this section to utilize the custom params instead when passing

[Kandeel4411]

There is also another bug here that is possible when you are passing the settings here:

drf-flex-fields/rest_flex_fields/serializers.py

Lines 102 to 107 in 25be9c1

	if type(serializer_class) == str:
	serializer_class = self._get_serializer_class_from_lazy_string(
	serializer_class
	)
	return serializer_class(**settings)

The serializer_class might not actually be a subclass of FlexFieldsSerializerMixin in which case passing the kwargs will result in a TypeError (again) since the __init__ method of ModelSerializer is explicitly defined (it has no extra kwargs)

I believe a check is needed here first to check if the serializer_class is an actual subclass of FlexFieldsSerializerMixin i.e:

 if name in nested_expand: 
    settings["expand"] = nested_expand[name] 
 
if name in nested_fields: 
    settings["fields"] = nested_fields[name] 
 
if name in nested_omit: 
    settings["omit"] = nested_omit[name] 
 
if type(serializer_class) == str: 
    serializer_class = self._get_serializer_class_from_lazy_string( 
        serializer_class 
    ) 
 
if issubclass(serializer_class, FlexFieldsSerializerMixin): 
    return serializer_class(**settings)
return serializer_class() 

[rsinger86]

Ah, nice catches. I'll get these in a new release soon.