Fix handling of responses with an empty body

Without this change the injector causes 'undefined' string to be returned (with no headers and not status code) instead of an empty response (for example 204: no content).

Wireshark dump:

DELETE /foo/bar/ HTTP/1.1
Host: localhost:3000
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive

undefined