Cookie header gets corrupted in actual request

```
In issue 42 we attempted to allow sending the cookie header in a request, but 
the implementation is flawed: a header like "Cookie: x=23" ends up getting sent 
as "Cookie: Cookie=x=23".
```

Original issue reported on code.google.com by `stephen....@gmail.com` on 4 Sep 2014 at 5:19
