first, relax. people go on appearance and conditioning. they don't know you, and she was attempting to be nice. the motivation was good, the discernment was poor. it's not about you. she doesn't know you.
second, if it ever happens again, just consider that the universe has given you a little something to pass along to someone who can really use it. gave it to *you* because you're more discerning and can find someone who actually needs it, unlike that lady.
getting offended at people for misguided kindness is a waste of your time.
it used to happen to my lover too, and it bugged him too. and he was white. "why do they always think i'm homeless?" he'd ask. who knows? maybe because his hair was long and he was in the park. but really, people generally don't look very carefully at what's around them, and that includes people. happens all the time, but not always with good intentions.