Add scheme to PoolKey and let port be None. (#84)

PoolKey calls unwrap() on an option that can be None. Specifically, the
local variable `port` can be None when PoolKey is constructed with a Url
whose scheme is unrecognized in url.port_or_known_default().

To fix that, make port an Option. Also, make scheme part of the PoolKey.
This prevents, for instance, a stream opened for `https://example.com:9999`
being reused on a request for `http://example.com:9999`.

Remove the test-only pool.get() accessor. This was used in only one test,
agent_pool in range.rs. This seemed like it was testing the agent more
than it was testing ranges, so I moved it to agent.rs and edited to
remove the range-testing parts.

Also, reject unrecognized URLs earlier in connect_socket so they don't
reach try_get_connection.

src/agent.rs

@@ -262,6 +262,7 @@ pub(crate) fn basic_auth(user: &str, pass: &str) -> String {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::io::Read;
 
     ///////////////////// AGENT TESTS //////////////////////////////
 
@@ -273,6 +274,35 @@ mod tests {
         });
     }
 
+    #[test]
+    #[cfg(any(feature = "tls", feature = "native-tls"))]
+    fn agent_pool() {
+        let agent = crate::agent();
+        let url = "https://ureq.s3.eu-central-1.amazonaws.com/sherlock.txt";
+        // req 1
+        let resp = agent.get(url).call();
+        assert!(resp.ok());
+        let mut reader = resp.into_reader();
+        let mut buf = vec![];
+        // reading the entire content will return the connection to the pool
+        reader.read_to_end(&mut buf).unwrap();
+
+        fn poolsize(agent: &Agent) -> usize {
+            let mut lock = agent.state.lock().unwrap();
+            let state = lock.as_mut().unwrap();
+            state.pool().len()
+        }
+        assert_eq!(poolsize(&agent), 1);
+
+        // req 2 should be done with a reused connection
+        let resp = agent.get(url).call();
+        assert!(resp.ok());
+        assert_eq!(poolsize(&agent), 0);
+        let mut reader = resp.into_reader();
+        let mut buf = vec![];
+        reader.read_to_end(&mut buf).unwrap();
+    }
+
     //////////////////// REQUEST TESTS /////////////////////////////
 
     #[test]