Incr bug fixes

I use django-redis-cache in my project, recently when found that when we do some incr / decr operation, we got some unexpected value,  such as we want to subtract 3 , but we got it add 1 ..

when I was debugging , I found that the reason was when there was a redid.ResponseError, value will be add 1, but I think it should add delta which was passed as a params. 

p.s.  the season caused this exception is when I set a long type integer to cache ,  the set function dumps it to a string , so when I do a decr operation, it would cause ResponseError . 

Sorry for my poor English , I hope I describe the issue clearly.