fix(edit): Format-by-clearing for keys/reprs, like decor

In this we were inconsistent with decor which we rely on just clearing
to get the default auto-formatted behavior.

crates/toml_edit/src/key.rs

@@ -123,7 +123,7 @@ impl Key {
 
     /// Auto formats the key.
     pub fn fmt(&mut self) {
-        self.repr = Some(to_key_repr(&self.key));
+        self.repr = None;
         self.decor.clear();
     }
 

crates/toml_edit/src/repr.rs

@@ -82,7 +82,7 @@ where
 
     /// Auto formats the value.
     pub fn fmt(&mut self) {
-        self.repr = Some(self.value.to_repr());
+        self.repr = None;
     }
 }