Avoid an additional allocation when logging errors

The call to `to_string()` results in an unnecessary allocation.  `Error`
only implements `ToString` through the `Display` trait.  The same
formatting logic is used, but calling `to_string()` causes an additional
heap allocation.

Author: jtgeibel

src/middleware/log_request.rs

@@ -43,7 +43,7 @@ impl Handler for LogRequests {
         };
 
         let error = if let Err(ref e) = res {
-            format!(" error=\"{}\"", e.to_string())
+            format!(" error=\"{}\"", e)
         } else {
             String::new()
         };