fix optional value

src/requests/get_user_profile_photos.rs

@@ -12,9 +12,11 @@ pub struct GetUserProfilePhotos<'a> {
     user_id: i32,
     /// Sequential number of the first photo to be returned. By default, all
     /// photos are returned.
+    #[serde(skip_serializing_if = "Option::is_none")]
     offset: Option<i64>,
     ///Limits the number of photos to be retrieved. Values between 1—100 are
     /// accepted. Defaults to 100.
+    #[serde(skip_serializing_if = "Option::is_none")]
     limit: Option<i64>,
 }
 
@@ -59,4 +61,13 @@ impl<'a> GetUserProfilePhotos<'a> {
         self.offset = Some(offset.into());
         self
     }
+
+    pub fn limit<T>(mut self, limit: T) -> Self
+        where
+            T: Into<i64>,
+    {
+        self.limit = Some(limit.into());
+        self
+    }
+
 }