Fix latest clippy warnings (#953)

src/message/dkim.rs

@@ -2,7 +2,6 @@ use std::{
     borrow::Cow,
     error::Error as StdError,
     fmt::{self, Display},
-    iter::IntoIterator,
     time::SystemTime,
 };
 

src/message/mailbox/types.rs

@@ -444,8 +444,6 @@ fn write_quoted_string_char(f: &mut Formatter<'_>, c: char) -> FmtResult {
 
 #[cfg(test)]
 mod test {
-    use std::convert::TryInto;
-
     use pretty_assertions::assert_eq;
 
     use super::Mailbox;

src/message/mimebody.rs

@@ -420,7 +420,6 @@ mod test {
     use pretty_assertions::assert_eq;
 
     use super::*;
-    use crate::message::header;
 
     #[test]
     fn single_part_binary() {

src/transport/smtp/extension.rs

@@ -4,7 +4,6 @@ use std::{
     collections::HashSet,
     fmt::{self, Display, Formatter},
     net::{Ipv4Addr, Ipv6Addr},
-    result::Result,
 };
 
 use crate::transport::smtp::{
@@ -292,14 +291,8 @@ impl Display for RcptParameter {
 
 #[cfg(test)]
 mod test {
-
-    use std::collections::HashSet;
-
     use super::*;
-    use crate::transport::smtp::{
-        authentication::Mechanism,
-        response::{Category, Code, Detail, Response, Severity},
-    };
+    use crate::transport::smtp::response::{Category, Code, Detail, Severity};
 
     #[test]
     fn test_clientid_fmt() {

src/transport/smtp/response.rs

@@ -5,7 +5,6 @@ use std::{
     fmt::{Display, Formatter, Result},
     result,
     str::FromStr,
-    string::ToString,
 };
 
 use nom::{