fix(client): retry when pool checkout returns closed HTTP2 connection (#2585)

When http2_only is true, we never try to open a new connection if there
is one open already, which means that if the existing connection that
gets checked out of the pool is closed, then the request won't happen.

src/error.rs

@@ -214,7 +214,7 @@ impl Error {
         &self.inner.kind
     }
 
-    fn find_source<E: StdError + 'static>(&self) -> Option<&E> {
+    pub(crate) fn find_source<E: StdError + 'static>(&self) -> Option<&E> {
         let mut cause = self.source();
         while let Some(err) = cause {
             if let Some(ref typed) = err.downcast_ref() {