Remove some warnings

Author: bold

Diff for: examples/smtp.rs

@@ -1,4 +1,4 @@
-use async_smtp::{EmailAddress, Envelope, SendableEmail, SmtpClient, Transport};
+use async_smtp::{EmailAddress, Envelope, SendableEmail, SmtpClient};
 
 fn main() {
     env_logger::init();

Diff for: examples/smtp_gmail.rs

@@ -1,5 +1,5 @@
 use async_smtp::smtp::authentication::Credentials;
-use async_smtp::{EmailAddress, Envelope, SendableEmail, SmtpClient, Transport};
+use async_smtp::{EmailAddress, Envelope, SendableEmail, SmtpClient};
 
 fn main() {
     async_std::task::block_on(async move {

Diff for: src/smtp/smtp_client.rs

@@ -411,7 +411,7 @@ impl<'a> SmtpTransport {
 
                 let client = std::mem::replace(&mut self.client, InnerClient::default());
                 let ssl_client = client.upgrade_tls_stream(tls_parameters).await?;
-                std::mem::replace(&mut self.client, ssl_client);
+                self.client = ssl_client;
 
                 debug!("connection encrypted");