write_redis: re-add colon dropped in b7984797

When running f3706b0b87, the following command gets sent to redis:
  "ZADD" "collectd/hostname/entropy/entropy" "1415487432.000000" "1415487432:932"
Meaning the value actually stored, and later returned by redis is: `<timstamp>:<value>`.

b7984797 accidentally dropped the comma separating the timestamp and the
value, which leads the plugin to store a somewhat confusing value in
redis:
  "ZADD" "collectd/hostname/entropy/entropy" "1415487432.000000" "1415487432932"