style(all): Style improvement with clippy

lettre/src/smtp/client/mod.rs

@@ -59,6 +59,7 @@ macro_rules! return_err (
     })
 );
 
+#[cfg_attr(feature = "cargo-clippy", allow(new_without_default_derive))]
 impl<S: Write + Read> Client<S> {
     /// Creates a new SMTP client
     ///

lettre/src/smtp/mod.rs

@@ -479,7 +479,7 @@ impl EmailTransport<SmtpResult> for SmtpTransport {
                         try_smtp!(
                             self.client.auth(
                                 mechanism,
-                                &self.client_info.credentials.as_ref().unwrap(),
+                                self.client_info.credentials.as_ref().unwrap(),
                             ),
                             self
                         );