well it was predicted by Galileo. He did experiments by doppin balls of different material off the tower at Pisa. it was actually verified by David Scott on the moon: https://www.youtube.com/watch?v=5C5_dOEyAfk
I don't know who may have first proposed the question but the specific experiment was conducted on the Moon in 1971, on the flight of Apollo 15. Astronaut David Scott dropped a falcon feather and a hammer and they hit the moon at the same time. http://en.wikipedia.org/wiki/Apollo_15