implement client ping (#108)

src/cmd/ping.rs

@@ -63,4 +63,17 @@ impl Ping {
 
         Ok(())
     }
+
+    /// Converts the command into an equivalent `Frame`.
+    ///
+    /// This is called by the client when encoding a `Ping` command to send
+    /// to the server.
+    pub(crate) fn into_frame(self) -> Frame {
+        let mut frame = Frame::array();
+        frame.push_bulk(Bytes::from("ping".as_bytes()));
+        if let Some(msg) = self.msg {
+            frame.push_bulk(Bytes::from(msg));
+        }
+        frame
+    }
 }