Resolve redundant_field_names clippy lint from PR 2448

    warning: redundant field names in struct initialization
        --> serde/src/private/ser.rs:1278:13
         |
    1278 |             map: map,
         |             ^^^^^^^^ help: replace it with: `map`
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_field_names
         = note: `-W clippy::redundant-field-names` implied by `-W clippy::all`

serde/src/private/ser.rs

@@ -1275,7 +1275,7 @@ where
 {
     fn new(map: &'a mut M) -> Self {
         FlatMapSerializeTupleVariantAsMapValue {
-            map: map,
+            map,
             fields: Vec::new(),
         }
     }