Who won the race on August 15?
CREATE TABLE "table1_28925058_1" (
  "round" real,
  "date" text,
  "grand_prix" text,
  "circuit" text,
  "pole_position" text,
  "fastest_lap" text,
  "race_winner" text
);
SELECT "race_winner" FROM "table1_28925058_1" WHERE "date"='August 15';