refactore: use clone_from() (#5451)

`a.clone_from(&b)` is equivalent to `a = b.clone()` in functionality,
but can be overridden to reuse the resources of a to avoid unnecessary
allocations.

src/mimeparser.rs

@@ -604,7 +604,7 @@ impl MimeMessage {
                 let mut filepart = self.parts.swap_remove(1);
 
                 // insert new one
-                filepart.msg = self.parts[0].msg.clone();
+                filepart.msg.clone_from(&self.parts[0].msg);
                 if let Some(quote) = self.parts[0].param.get(Param::Quote) {
                     filepart.param.set(Param::Quote, quote);
                 }