[Weather] use IsNaN instead of comparison (#2588)

Author: xieofxie

skills/csharp/experimental/weatherskill/Dialogs/ForecastDialog.cs

@@ -66,7 +66,7 @@ private async Task<DialogTurnResult> RouteToGeographyPromptOrForecastResponse(Wa
             var state = await _stateAccessor.GetAsync(stepContext.Context);
             var geography = state.Geography;
 
-            if (string.IsNullOrEmpty(geography) && state.Latitude == double.NaN)
+            if (string.IsNullOrEmpty(geography) && double.IsNaN(state.Latitude))
             {
                 return await stepContext.NextAsync();
             }
@@ -122,7 +122,7 @@ private async Task<DialogTurnResult> GetWeatherResponse(WaterfallStepContext ste
             {
                 state.GeographyLocation = await service.GetLocationByQueryAsync(state.Geography);
             }
-            else if (state.Latitude != double.NaN)
+            else if (!double.IsNaN(state.Latitude))
             {
                 state.GeographyLocation = await service.GetLocationByGeoAsync(state.Latitude, state.Longitude);
             }

skills/csharp/experimental/weatherskill/Models/SkillState.cs

@@ -7,6 +7,11 @@ namespace WeatherSkill.Models
 {
     public class SkillState
     {
+        public SkillState()
+        {
+            Clear();
+        }
+
         public string Token { get; internal set; }
 
         public WeatherSkillLuis LuisResult { get; internal set; }