Retry some pooled connections failing when server closes. Close #10

This is not a perfect solution. It works as long as we are not sending
any body bytes. We discover the error first when attempting to read
the response status line. That means we discover the error after
sending body bytes. To be able to re-send the body, we would need to
introduce a buffer to be able to replay the body on the next
request. We don't currently do that.

src/test/agent_test.rs

@@ -51,3 +51,29 @@ fn agent_cookies() {
 
     agent.get("test://host/agent_cookies").call();
 }
+
+#[test]
+fn connection_reuse() {
+    use std::io::Read;
+    use std::time::Duration;
+
+    let agent = Agent::default().build();
+    let resp = agent.get("https://fau.xxx/").call();
+
+    // use up the connection so it gets returned to the pool
+    assert_eq!(resp.status(), 200);
+    resp.into_reader().read_to_end(&mut vec![]).unwrap();
+
+    // wait for the server to close the connection. fau.xxx has a
+    // 2 second connection keep-alive. then it closes.
+    std::thread::sleep(Duration::from_secs(3));
+
+    // try and make a new request on the pool. this fails
+    // when we discover that the TLS connection is dead
+    // first when attempting to read from it.
+    let resp = agent.get("https://fau.xxx/").call();
+    if let Some(err) = resp.synthetic_error() {
+        panic!("Pooled connection failed! {:?}", err);
+    }
+    assert_eq!(resp.status(), 200);
+}