add Proxy::custom to allow custom proxy functions

src/into_url.rs

@@ -36,3 +36,7 @@ impl<'a> PolyfillTryInto for &'a String {
 pub fn to_uri(url: &Url) -> ::hyper::Uri {
     url.as_str().parse().expect("a parsed Url should always be a valid Uri")
 }
+
+pub fn to_url(uri: &::hyper::Uri) -> Url {
+    uri.as_ref().parse().expect("reqwest Uris should only ever come from Urls")
+}