[MRG] Do not set Referer by default when its value is None #475
I'd prefer an even more "functional-style" test - e.g. check that Referer header is not set when None is used and is set if None is not used (no explicit Referer header and explicit empty Referer header). Checking for setdefault looks like checking against implementation details.
As for the docs, it seems that "unset" would be a bit misleading - we can't unset a header in Request constructor because the header can be added later by a middleware, and the feature should still work. It is more like "preventing header from being sent".