MAX_INCLUDE_DEPTH include parsing for querystring is wrong

[iamareebjamal]

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Let's take a look at this code

https://github.com/miLibris/flask-rest-jsonapi/blob/b44bc08b11213d49fadae873650d3555889052ec/flask_rest_jsonapi/querystring.py#L193-L201

https://github.com/miLibris/flask-rest-jsonapi/blob/b44bc08b11213d49fadae873650d3555889052ec/flask_rest_jsonapi/querystring.py#L193

This expects self.querystring['include'] to be a list, whereas we know that it is a string by looking at last line

https://github.com/miLibris/flask-rest-jsonapi/blob/b44bc08b11213d49fadae873650d3555889052ec/flask_rest_jsonapi/querystring.py#L201

Hence, this block of code is wrong.

https://github.com/miLibris/flask-rest-jsonapi/blob/b44bc08b11213d49fadae873650d3555889052ec/flask_rest_jsonapi/querystring.py#L195-L199

If MAX_INCLUDE_DEPTH is not None and querystring is not empty, it'll loop through the individual characters of the querystring and not different parts of include string separated by commas