Remove DEFAULT_HOST (#153)

In a few places we relied on "localhost" as a default if a URL's host
was not set, but I think it's better to error out in these cases.

In general, there are a few places in Unit that assumed there is a
host as part of the URL. I've made that explicit by doing a check
at the beginning of `connect()`. I've also tried to plumb through
the semantics of "host is always present" by changing the parameter
types of some of the functions that use the hostname.

I considered a more thorough way to express this with types - for
instance implementing an `HttpUrl` struct that embeds a `Url`, and
exports most of the same methods, but guarantees that host is always
present. However, that was more invasive than this so I did a smaller
change to start.

src/request.rs

@@ -12,7 +12,6 @@ use crate::body::BodySize;
 use crate::body::{Payload, SizedReader};
 use crate::error::Error;
 use crate::header::{self, Header};
-use crate::pool;
 use crate::unit::{self, Unit};
 use crate::Response;
 
@@ -500,8 +499,13 @@ impl Request {
     /// assert_eq!(req2.get_host().unwrap(), "localhost");
     /// ```
     pub fn get_host(&self) -> Result<String, Error> {
-        self.to_url()
-            .map(|u| u.host_str().unwrap_or(pool::DEFAULT_HOST).to_string())
+        match self.to_url() {
+            Ok(u) => match u.host_str() {
+                Some(host) => Ok(host.to_string()),
+                None => Err(Error::BadUrl("No hostname in URL".into())),
+            },
+            Err(e) => Err(e),
+        }
     }
 
     /// Returns the scheme for this request.
@@ -637,3 +641,13 @@ impl fmt::Debug for TLSConnector {
         f.debug_struct("TLSConnector").finish()
     }
 }
+
+#[test]
+fn no_hostname() {
+    let req = Request::new(
+        &Agent::default(),
+        "GET".to_string(),
+        "unix:/run/foo.socket".to_string(),
+    );
+    assert!(req.get_host().is_err());
+}