Fix bug: Search optional params are not applied when they are a list of strings

[eskombro]

When a search parameter like attributesToHighlight or attributesToRetrieve are given as a list of strings, they should be encoded as a comma separated string into the url:

attributesToRetrieve = ["some", "string"]

should become

... &attributesToRetrieve=some,string ...

This encoding is not the default behaviour of the url lib encoding and it was introducing the [ and ] characters in the url (and some +) so MeiliSearch was ignoring those parameters.

Before encoding this parameters, I added a few lines:

for key in opt_params:
    if isinstance(opt_params[key], list):
        opt_params[key] = ",".join(opt_params[key])

this will transform the lists of strings in the expected format.

A test was added to the meilisearch/tests/index/test_index_document_meilisearch.py file to check this behavior

Closes #85

[curquiza]

Top!
Could you put single quotes everwhere to stay consistent? 🙂

[eskombro]

Done @curquiza ! (and for all the tests too)

[curquiza]

👍