Remove JsonIgnores, require opt-in properties

osu.Game.Rulesets.Osu/Difficulty/OsuDifficultyAttributes.cs

@@ -30,16 +30,12 @@ namespace osu.Game.Rulesets.Osu.Difficulty
         [JsonProperty("overall_difficulty")]
         public double OverallDifficulty { get; set; }
 
-        [JsonIgnore]
         public double DrainRate { get; set; }
 
-        [JsonIgnore]
         public int HitCircleCount { get; set; }
 
-        [JsonIgnore]
         public int SliderCount { get; set; }
 
-        [JsonIgnore]
         public int SpinnerCount { get; set; }
 
         public override IEnumerable<(int attributeId, object value)> ToDatabaseAttributes()

osu.Game/Rulesets/Difficulty/DifficultyAttributes.cs

@@ -9,12 +9,11 @@ using osu.Game.Rulesets.Mods;
 
 namespace osu.Game.Rulesets.Difficulty
 {
+    [JsonObject(MemberSerialization.OptIn)]
     public class DifficultyAttributes
     {
-        [JsonIgnore]
         public Mod[] Mods { get; set; }
 
-        [JsonIgnore]
         public Skill[] Skills { get; set; }
 
         [JsonProperty("star_rating", Order = -3)]