Clear Content-Length header from redirected requests (#394)

This PR removes the Content-Length header from subsequent redirect requests if set.
A test verifies the new behaviour.

src/unit.rs

@@ -205,6 +205,8 @@ pub(crate) fn connect(
         debug!("redirect {} {} -> {}", resp.status(), url, new_url);
         history.push(unit.url.to_string());
         body = Payload::Empty.into_read();
+        unit.headers.retain(|h| h.name() != "Content-Length");
+
         // recreate the unit to get a new hostname and cookies for the new host.
         unit = Unit::new(
             &unit.agent,