Add 'setEmptyBody' configuration option and functionality, to allow s…

[tiagojsag]

…etting an empty body when no supported content-type is provided

This fixes https://github.com/koajs/koa-body/issues/218 and extends https://github.com/koajs/koa-body/pull/219

Checklist

• [x] I have ensured my pull request is not behind the main or master branch of the original repository.
• [x] I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• [ ] I have written a commit message that passes commitlint linting.
• [x] I have ensured that my code changes pass linting tests.
• [x] I have ensured that my code changes pass unit tests.
• [x] I have described my pull request and the reasons for code changes along with context if necessary.