fastcig: GET requests send along the body (#1975)

Fixes #1961

According to RFC 7231 and RFC 7230, there's
no reason a GET-Request can't have a body
(other than it possibly not being supported
by existing software). It's use is simply not
defined, and is left to the application.
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500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f