Fix a few clippy warnings (#579)

src/message/mimebody.rs

@@ -175,7 +175,7 @@ fn make_boundary() -> String {
 
 impl MultiPartKind {
     fn to_mime<S: Into<String>>(&self, boundary: Option<S>) -> Mime {
-        let boundary = boundary.map_or_else(make_boundary, |s| s.into());
+        let boundary = boundary.map_or_else(make_boundary, Into::into);
 
         format!(
             "multipart/{}; boundary=\"{}\"{}",

src/transport/smtp/commands.rs

@@ -55,7 +55,7 @@ impl Display for Mail {
         write!(
             f,
             "MAIL FROM:<{}>",
-            self.sender.as_ref().map(|s| s.as_ref()).unwrap_or("")
+            self.sender.as_ref().map_or("", |s| s.as_ref())
         )?;
         for parameter in &self.parameters {
             write!(f, " {}", parameter)?;

src/transport/smtp/error.rs

@@ -76,8 +76,7 @@ impl Error {
     /// Returns the status code, if the error was generated from a response.
     pub fn status(&self) -> Option<Code> {
         match self.inner.kind {
-            Kind::Transient(code) => Some(code),
-            Kind::Permanent(code) => Some(code),
+            Kind::Transient(code) | Kind::Permanent(code) => Some(code),
             _ => None,
         }
     }

src/transport/smtp/response.rs

@@ -122,7 +122,7 @@ impl Code {
     }
 
     /// Tells if the response is positive
-    pub fn is_positive(&self) -> bool {
+    pub fn is_positive(self) -> bool {
         matches!(
             self.severity,
             Severity::PositiveCompletion | Severity::PositiveIntermediate