"rag-head N. Amer. slang, one who wears a turban or cloth about the hair"
"1921 Dialect Notes V. 111 *Raghead, a Hindu; any Asiatic. From the turbanned Asiatics who are common on the campus [of the University of California]. 1970 C. MAJOR Dict. Afro-Amer. Slang 96 Raghead, black male who wears a scarf tied around his head to protect an expensive hairdo. 1975 Canadian Mag. 8 Mar. 6/1 East Indians are called ‘rag-heads’ if they continue to wear the traditional turban of the Sikh religion."
I don't see any refernce to Islam there, although there are several references to race. But then again, it's only the most authoritative source of the history of the English language, so what does it know?
Your understanding of "race" is unsurprisingly confused. A race is an anthropological classification of heredity - like black, caucasian, arab etc.
Aside from the "black male" reference, which is to an antiquated and unrelated use of the rerm "raghead," the terms quoted in that definition are not races. They are references to national origins (East Indians), religious groups (Hindus, Sikhs), and geographical populations (Asiatics).
The unifying point of the definition is its reference to the cloth headgarb connoted in the term itself. As that headgarb has, in recent decades, become increasingly associated with the mahometan religion, the term has taken on the characteristic of an anti-mahometan remark regardless of the mahometan's race.
Will the OED suffice?
LOL. Well found, sir.