Adding possibility to send body null and tests

[alallema]

Description
Adding the possibility for the client to send body null.
Fixing update settings method.
Adding tests to check if nil could be sent properly.

Issue related
#121

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Still not working on update_synonyms() and update_stop_words() method because the meilisearch API doesn't seem to take null as arguments

[curquiza]

I confirm, the synonyms and the stop words cannot be updated with null via the MeiliSearch API. I will open an issue to fix this in transplant once all the settings are implemented in.

Waiting for the approval of @bidoubiwa before any merge

[curquiza]

2 remarks:

• your issue was not linked with this PR, even if you mentioned it in the description. I added it 😉 It's important so that the issue is closed automatically by GitHub once the PR is merged. Sometimes (depending on how you mention it and how many issues you mention) GitHub does not link the issue in the PR, unfortunately you have to check every time 😅
• The linter fails: this section in the CONTRIBUTING might help you fix it: https://github.com/meilisearch/meilisearch-ruby/blob/main/CONTRIBUTING.md#linter

[curquiza]

🚀

[bidoubiwa]

🔥🔥🔥

[bidoubiwa]

bors merge

[bors]

Build succeeded:

• integration-tests (ruby 2.6)
• integration-tests (ruby 2.7)
• linter-check