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/unit.rs

@@ -291,6 +291,10 @@ pub(crate) fn combine_query(url: &Url, query: &QString, mix_queries: bool) -> St
 
 /// Connect the socket, either by using the pool or grab a new one.
 fn connect_socket(unit: &Unit, use_pooled: bool) -> Result<(Stream, bool), Error> {
+    match unit.url.scheme() {
+        "http" | "https" | "test" => (),
+        _ => return Err(Error::UnknownScheme(unit.url.scheme().to_string())),
+    };
     if use_pooled {
         let state = &mut unit.agent.lock().unwrap();
         if let Some(agent) = state.as_mut() {