Handle Dovecot's periodic IDLE notification messages ("OK Still here").

These messages show up at an interval controlled by Dovecot's
imap_idle_notify_interval setting, which defaults to 2 minutes.
These messages were causing the IDLE loop to exit prematurely
because it looks like a notification response from the server.

src/extensions/idle.rs

@@ -93,16 +93,26 @@ impl<'a, T: Read + Write + 'a> Handle<'a, T> {
     /// This is necessary so that we can keep using the inner `Session` in `wait_keepalive`.
     fn wait_inner(&mut self) -> Result<()> {
         let mut v = Vec::new();
+        loop {
             match self.session.readline(&mut v).map(|_| ()) {
                 Err(Error::Io(ref e))
-                if e.kind() == io::ErrorKind::TimedOut || e.kind() == io::ErrorKind::WouldBlock =>
+                    if e.kind() == io::ErrorKind::TimedOut
+                        || e.kind() == io::ErrorKind::WouldBlock =>
                 {
                     // we need to refresh the IDLE connection
                     self.terminate()?;
                     self.init()?;
-                self.wait_inner()
+                    return self.wait_inner();
                 }
                 r => r,
+            }?;
+
+            // Handle Dovecot's imap_idle_notify_interval message
+            if v.eq_ignore_ascii_case(b"* OK Still here\r\n") {
+                v.clear();
+            } else {
+                break Ok(());
+            }
         }
     }