Move preference to query params

[afg419]

This PR moves the preference parameter from the query body to the url as a query parameter. This is in accord with elasticsearch 7 specification.

Current behavior:

PostIndex.preference(:_local).wrappers
#=> Elasticsearch::Transport::Transport::Errors::BadRequest ([400] {"error":{"root_cause":[{"type":"parsing_exception","reason":"Unknown key for a VALUE_STRING in [preference].","line":1,"col":15}],"type":"parsing_exception","reason":"Unknown key for a VALUE_STRING in [preference].","line":1,"col":15},"status":400})

New behavior post PR:

PostIndex.preference(:_local).wrappers
#=> stuff

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[rabotyaga]

Hey @afg419 👋 !
Thanks for the contribution!
Do you mind adding a CHANGELOG line?

[afg419]

Hey @afg419 👋 !
Thanks for the contribution!
Do you mind adding a CHANGELOG line?

Have done! And thanks for the review.