[Elixir] Adds workaround for httpc for post/put/patch requests

[mrmstn]

This PR will fix #5403 by adding an empty string if no body was defined for requests with a method of POST, PATCH or PUT.

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  cc: @wing328

[mrmstn]

Thins I need to check before merging:
What about

• file uploads
• optional body params

[mrmstn]

I've changed the way to handle this.. now, the generator looks for a required body params (forms included). If one is found, the patch doesn't need to take place.

The patch itself will simply look if a body param was set by one of the optional body params. If the body is still not present or nil, it it will be replaces by an empty string.

@wing328 Could you do me the honor and give me a short review on this PR? :)

[wing328]

LGTM. Local tests passed as well.