requests: check stop chan if fail to read from response body

The failure on reading from response body may be caused because user
has asked to cancel http requests. So we check the stop chan status
to return correct error.

#146
@marc-barry: I cannot replay your case that it cannot stop requests. I am using the +git version of etcd and go-etcd. I hope this one could help.
fixes coreos/etcd#901
@onsi: It fixes the wrong error problem in your script well.