Our HTTPRequest/HTTPResponse's .content attribute can currently be either

• None, if we have not read the body yet (e.g. in the reponseheaders hook)
• CONTENT_MISSING=0, if we deliberately deleted the body
• b"", if we read an (empty) body.
• b"some-content", if we actually read some content.

While the first three values intuitively evaluate to False, I feel the bytes/int/None heterogenity a bit confusing. I'd like to combine the first two cases, so that the type of this attribute can accurately be described as Optional[bytes]. This would fix #692 and make the code IMO easier to understand. We could also first deprecate CONTENT_MISSING gradually by setting it to None, and then remove it later on. @cortesi, would you be okay with this approach?