add 304 cache controll base on content hash

fix https://github.com/sintaxi/harp/issues/300

set etag base on body md5 hash
if equal return 304 instead of re-send the file

this is a temporary solution for cache controll
for there is no good cache method, detail in https://github.com/sintaxi/harp/pull/79

i don't know if there is a good idea for cache