Take into account only not skipped flatten fields when choose serialization form

Consequence: `FlattenSkipSerializing`
- uses `serialize_struct` instead of `serialize_map`

serde_derive/src/ser.rs

@@ -297,7 +297,10 @@ fn serialize_struct(params: &Parameters, fields: &[Field], cattrs: &attr::Contai
         u32::MAX
     );
 
-    if cattrs.has_flatten() {
+    let has_non_skipped_flatten = fields
+        .iter()
+        .any(|field| field.attrs.flatten() && !field.attrs.skip_serializing());
+    if has_non_skipped_flatten {
         serialize_struct_as_map(params, fields, cattrs)
     } else {
         serialize_struct_as_struct(params, fields, cattrs)