Support for a list of expansions with multiple expand parameters instead of separated by a comma.

[SorianoMarmol]

Hello. First of all, thank you for your work and effort with the library!

We have noticed that, despite expansions working correctly with multiple expansion parameters:

expand=field1&expand=field2 instead of ?expand=field1,field2

the 'is_expanded' function does not respond appropriately due to the use of 'request.query_params.get(EXPAND_PARAM).'
In the case of receiving a list, it only returns one of the values.

However, the 'getlist' method does return both fields.
I illustrate it with an example:

# expand=field1&expand=field2
ipdb> self.request.query_params
<QueryDict: {'expand': ['asset', 'positions']}>
ipdb> self.request.query_params.getlist('expand')
['field1', 'field2']
ipdb> self.request.query_params.get('expand')
'field2'

In this case, it's already a list.

Would it be possible to manage this scenario as well?

[SorianoMarmol]

Something like that

def is_expanded(request, field: str) -> bool:
    """ Examines request object to return boolean of whether
        passed field is expanded.
    """
    expand_value = request.query_params.getlist(EXPAND_PARAM)
    if not expand_value:
        return False
    expand_fields = []
    if len(expand_value) > 1:  # already a list
        expand_params = expand_value
    else:
        expand_params = expand_value[0].split(",")
    for f in expand_params:
        expand_fields.extend([_ for _ in f.split(".")])

    return any(field for field in expand_fields if field in WILDCARD_VALUES) or field in expand_fields