Update error.go

this caused `http: multiple response.WriteHeader calls` when using method `CustomAbort` or `Abort` when status is already in errMap like 404.

error.go

@@ -453,7 +453,6 @@ func exception(errcode string, ctx *context.Context) {
 
 func executeError(err *errorInfo, ctx *context.Context, code int) {
 	if err.errorType == errorTypeHandler {
-		ctx.ResponseWriter.WriteHeader(code)
 		err.handler(ctx.ResponseWriter, ctx.Request)
 		return
 	}