From<http::Response> for Response (#360)

This adds an implementation to convert a `Response` type from the `http`
crate to the `async_impl::Response` type. This is the first step to
allow us to convert `http::Response` objects to `request::Response`
objects

This also adds an extension trait for the `http::response::Builder`
type. The `http::Response` object does not provide a way to access the
"final" url that the response is derived from, so we can't easily
provide that in the `From<http::Response>` implementation. For users who
are manually constructing `http::Response` objects for use in tests,
etc, they can import this extension trait, which adds a `.url()` builder
method that will allow them to pass a `Url`, which we then convert to
our newtype'd Url and add to the `http::Response`'s `extensions`. Then,
when converting from `http::Response` to `async_impl::Response` we can
pull that value out of the `extensions` and use it to construct the
`async_impl::Response`

Closes #333

src/client.rs

@@ -500,7 +500,7 @@ impl ClientHandle {
             }
         };
         res.map(|res| {
-            response::new(res, self.timeout.0, KeepCoreThreadAlive(self.inner.clone()))
+            response::new(res, self.timeout.0, KeepCoreThreadAlive(Some(self.inner.clone())))
         })
     }
 }
@@ -517,4 +517,10 @@ impl Default for Timeout {
 
 // pub(crate)
 
-pub struct KeepCoreThreadAlive(Arc<InnerClientHandle>);
+pub struct KeepCoreThreadAlive(Option<Arc<InnerClientHandle>>);
+
+impl KeepCoreThreadAlive {
+    pub(crate) fn empty() -> KeepCoreThreadAlive {
+        KeepCoreThreadAlive(None)
+    }
+}