[Elixir] Fixes optional body mapping

[mrmstn]

Closes #5234

This PR will map an optional body param, given by the static key body to the request body. This makes it relatively transparent to read what actually happens, instead of "mapping the title" in the optional param. This is also the default behavior for primitiv types.

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  cc: @wing328 I'm sorry it's you again 🙈

[mrmstn]

Hmm... I've no idea why the pipeline failed, could someone trigger the Shippable Action again?

[wing328]

@mrmstn restarted and all tests passed 👍

cc: @wing328 I'm sorry it's you again 🙈

No problem. Thanks again for the PR. Have a nice weekend.