Remove Headers from the public API. (#224)

It turns out Headers is actually an internal-only API. None of the
user-facing types use it.

Unfortunately, making it unexported also required deleting the doctests,
since doctests can only run against a public interface.

src/unit.rs

@@ -7,16 +7,14 @@ use url::Url;
 #[cfg(feature = "cookies")]
 use cookie::Cookie;
 
+use crate::body::{self, BodySize, Payload, SizedReader};
 use crate::error::{Error, ErrorKind};
 use crate::header;
+use crate::header::{get_header, Header};
 use crate::resolve::ArcResolver;
+use crate::response::Response;
 use crate::stream::{self, connect_test, Stream};
 use crate::Agent;
-use crate::{
-    body::{self, BodySize, Payload, SizedReader},
-    header::get_header,
-};
-use crate::{Header, Response};
 
 /// A Unit is fully-built Request, ready to execute.
 ///