convert return values of type []byte from the redis driver into string automatically before returning the value to the user of the cache system

- this seems to be happening very easily with the GET command on a redis setup on windows, the return value is an array of the ascii code of the string if not converted automatically
- tested and verified the fix on windows 7, redis 2.8 and the latest version of github.com/garyburd/redigo/redis
Merge branch 'develop' of https://github.com/astaxie/beego into develop
Merge branch 'develop' of https://github.com/haowang1013/beego into develop