Fixes RoleTypeReader (#631)

src/Discord.Net.Commands/Readers/RoleTypeReader.cs

@@ -31,7 +31,7 @@ namespace Discord.Commands
                     AddResult(results, role as T, role.Name == input ? 0.80f : 0.70f);
 
                 if (results.Count > 0)
-                    return Task.FromResult(TypeReaderResult.FromSuccess(results.Values));
+                    return Task.FromResult(TypeReaderResult.FromSuccess(results.Values.ToReadOnlyCollection()));
             }
             return Task.FromResult(TypeReaderResult.FromError(CommandError.ObjectNotFound, "Role not found."));
         }