Fix clippy for rust 1.52

There are new lints are added in clippy for rust 1.52

Signed-off-by: Tim Zhang <tim@hyper.sh>

src/error.rs

@@ -76,6 +76,7 @@ impl fmt::Display for Error {
 
 impl StdError for Error {
     fn cause(&self) -> Option<&dyn StdError> {
+        #[allow(clippy::manual_map)]
         match self.cause {
             Some(ref x) => Some(&**x),
             None => None,