Adding System.IO and System.Threading.Tasks to the list of default imports.

Author: fabiocav

src/WebJobs.Script/Description/CSharp/FunctionMetadataResolver.cs

@@ -56,8 +56,10 @@ public sealed class FunctionMetadataResolver : MetadataReferenceResolver
             {
                 "System",
                 "System.Collections.Generic",
+                "System.IO",
                 "System.Linq",
                 "System.Net.Http",
+                "System.Threading.Tasks",
                 "Microsoft.Azure.WebJobs",
                 "Microsoft.Azure.WebJobs.Host"
             };