Suppose a Muslim wished you a "Happy Ramadan"?
I bet you think that as a Christian person, I would get all offended and stomp my feet and go off in a huff, thinking, "Why, How Dare HE! He knows I am a Christian! How offensive!"
If you think that, my friend, it says more about your own prejudices than it does about me.
For in fact, if a Musline were to wish me "Happy Ramadan", I would quite graciously say to him or her, "Why, thank you, my friend. Thanks for wishing me an entire month of happiness. May you also enjoy a most happy Ramadan as well."
To take offense when none was intended seems to me to be really quite petty and mean-spirited.