Updated OpenApiWalker to include walking encoding headers Issue #405

Author: darrelmiller

src/Microsoft.OpenApi/Services/OpenApiWalker.cs

@@ -732,6 +732,11 @@ internal void Walk(OpenApiEncoding encoding)
             }
 
             _visitor.Visit(encoding);
+
+            if (encoding.Headers != null)
+            {
+                Walk(encoding.Headers);
+            }
             Walk(encoding as IOpenApiExtensible);
         }