Fix struct_variant serialization to match serde_json's output

Author: MabezDev

src/ser/mod.rs

@@ -323,10 +323,12 @@ where
         self,
         _name: &'static str,
         _variant_index: u32,
-        _variant: &'static str,
+        variant: &'static str,
         _len: usize,
     ) -> Result<Self::SerializeStructVariant> {
-        self.buf.push(b'{')?;
+        self.buf.extend_from_slice(b"{\"")?;
+        self.buf.extend_from_slice(variant.as_bytes())?;
+        self.buf.extend_from_slice(b"\":{")?;
 
         Ok(SerializeStructVariant::new(self))
     }
@@ -622,6 +624,6 @@ mod tests {
         }
         let a = A::A { x: 54, y: 720 };
 
-        assert_eq!(&*crate::to_string::<N, _>(&a).unwrap(), r#"{"x":54,"y":720}"#);
+        assert_eq!(&*crate::to_string::<N, _>(&a).unwrap(), r#"{"A":{"x":54,"y":720}}"#);
     }
 }

src/ser/struct_.rs

@@ -97,7 +97,7 @@ where
     }
 
     fn end(self) -> Result<Self::Ok> {
-        self.ser.buf.push(b'}')?;
+        self.ser.buf.extend_from_slice(b"}}")?;
         Ok(())
     }
 }