Since HttpResponse is not *only* used for XSS, it is still valuable to know the
content is send as part of the response.
The *proper* solution to this problem of not all HttpResponses being vulnerable
to XSS is probably to define a new abstract class in Http.qll called
HttpResponseXSSVulnerableSink (or similar). I would like to model a few more
libraries/frameworks before fully comitting to an approach though.