Fix a couple of feature flag uses

Wrap a json doc-test in cfg block to stop in failing when tests are
run with --no-default-features

src/unit.rs

@@ -16,6 +16,7 @@ use crate::{Error, Header, Request, Response};
 #[cfg(feature = "tls")]
 use crate::request::TLSClientConfig;
 
+#[cfg(feature = "cookie")]
 use crate::pool::DEFAULT_HOST;
 
 /// It's a "unit of work". Maybe a bad name for it?