Use const instead of fixed value

[anastygnome]

Hey, while reviewing the code, I found this TODO that seemed like it was forgotten.

[gazpachoking]

Good call. Originally I meant a constant that specified what the default merge mode of datastar was, but that only mattered when the constants were being generated automatically by d*. If the default ever changed, (which seems highly unlikely,) we would need to update the SDK no matter what now.

Thanks!