Fix another bug

Author: MaggieKimani1

src/Microsoft.OpenApi/Models/OpenApiHeader.cs

@@ -19,7 +19,7 @@ namespace Microsoft.OpenApi.Models
     /// </summary>
     public class OpenApiHeader : IOpenApiSerializable, IOpenApiReferenceable, IOpenApiExtensible, IEffective<OpenApiHeader>
     {
-        private JsonSchema _schema;
+        protected JsonSchema _schema;
 
         /// <summary>
         /// Indicates if object is populated with data or is just a reference to the data

src/Microsoft.OpenApi/Models/References/OpenApiHeaderReference.cs

@@ -73,7 +73,7 @@ public override string Description
         public override bool AllowEmptyValue { get => Target.AllowEmptyValue; set => Target.AllowEmptyValue = value; }
 
         /// <inheritdoc/>
-        public override JsonSchema Schema { get => Target.Schema; set => Target.Schema = value; }
+        public override JsonSchema Schema { get => _schema; set => _schema = value; }
 
         /// <inheritdoc/>
         public override ParameterStyle? Style { get => Target.Style; set => Target.Style = value; }