Don't return empty string for .json() if Transfer-Encoding is chunked

[davidrunger]

fixes #463

Here is some relevant documentation about the Transfer-Encoding header: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Transfer-Encoding , specifically when a chunked value is set for that header:

chunked: Data is sent in a series of chunks. The Content-Length header is omitted in this case [...] [emphasis added]

This is in contradiction to an implicit assumption seemingly made in #459 that a Content-Length header will always be set to some non-zero value if there is content in the response body.

The logic change in this PR returns a blank string for .json() only if the Content-Length header is '0' or missing (per #459) AND if there is not a Transfer-Encoding header with a value of chunked.

The test added in this PR sets a Transfer-Encoding: chunked header (and omits a Content-Length header) via the following setup, which writes to the response in chunks rather than all at once:

server.get('/', async (request, response) => {
  response.write('[');
  response.write('"one",');
  response.write('"two"');
  response.end(']');
});

[sindresorhus]

Thanks for fixing :)