URL.include_query_params overwrites query string parameters with the same name

[lexabug]

I've noticed that the URL.include_query_params() overwrites query string parameters with the same name. For example:

from starlette.datastructures import URL, MultiDict

url = URL('my_test_url_example')
query_params = MultiDict([('my_id', '143155'), ('language', 'en'), ('list_p[]', 'item1'), ('list_p[]', 'original_name'), ('list_p[]', 'item_3')])
new_url = url.include_query_params(**query_params)

So the new_url will be:

URL('my_url_example?my_id=143155&language=en&list_p%5B%5D=item_3')

Which is incorrect. The specification of the URLs allows multiple query string parameters with the same name, so the backend that should process that query string must correctly treat such parameters as arrays/lists.

[jhominal]

I think that this issue is due to the way that, as MultiDict is unpacked with **, that call basically makes it so that that call is equivalent to:

new_url = url.include_query_params(**dict(query_params.items()))

Basically, MultiDict implements the Mapping interface by restricting the Mapping interface to accessing a single item for each key. That is for ease of use (in the very common case where each query string key only has a single value), but it does fail in this kind of usage.

Do we want to add a provision for that kind of usage? E.g. there could be an overload:

def include_query_params(self, *items: typing.Union[
            ImmutableMultiDict[str, str]",
            typing.Mapping[str, str],
            typing.Iterable[typing.Tuple[str, str]],
        ],
): ...

[lexabug]

That would be great!

[lexabug]

Here is the PR for that #1738