Fix bug [python][client] generated python client code cannot POST object in multipart/form-data

[itaru2622]

as described in #8068 , encode object to json and add 'content-type:application/json' for sending multipart/form-data.
and checked if the request is sent to server with collect format described in #8068.

fix #8068

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[itaru2622]

I should fix my mistake about line 259, as below:

        if collection_types is None:
-          collection_types = (dict,list)
+          collection_types = (dict)

[itaru2622]

the above is fixed by a6c8d1a.

[spacether]

This looks great, thank you for the PR and the update!