Search parametters don't accept lists of strings (and should)

[eskombro]

While working in the improve tests PR, #80, we realized that the search parameters like attributesToRetrieve can't handle a list of strings. you should be able to pass a list of fields, not just a single field or '*'.

The test we did and failed was:

    def test_basic_search_params_with_string_list(self):
        """Tests search with string list in query params"""
        response = self.index.search(
            'a',
            {
                'limit': 5,
                'attributesToRetrieve': ['title', 'overview'],
            }
        )
        assert isinstance(response, object)
        assert len(response['hits']) == 5
        assert "title" in response['hits'][0]
        assert "overview" in response['hits'][0]
        assert not "release_date" in response['hits'][0]

And the error shows that MeiliSearch response is sending all the fields

E       AssertionError: assert not 'release_date' in {'id': '537915', 'overview': 'A young woman falls for a guy with a dark secret and the two embark on a rocky relationship.', 'poster': 'https://image.tmdb.org/t/p/w1280/u3B2YKUjWABcxXZ6Nm9h10hLUbh.jpg', 'release_date': 1554944400, ...}

THIS TEST SHOULD BE ADDED WHEN ISSUE IS FIXED