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.
@@ -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()
};
