fix: infer media-type for falsy bodies except 'undefined'

[fpoppinga]

This fixes a bug, where the Content-Type header would be reset to the default, when the request.body contains a falsy value.

For example, given using the fetch api, when sending a request with body 0 (i.e. the number zero), the Content-Type header would default to text/plain; utf-8 (which is the browser default), even when application/json would be expected (and would be set if the request body was e.g. 1, i.e. the number one).