Send content-length instead of chunked transfer

With this the results look like:

```
Python 2.7.6
go version go1.3.3 darwin/amd64
BENCH SOCKET:
   8GiB 0:00:19 [ 424MiB/s] [======================================================>] 100%
BENCH HTTPLIB:
   8GiB 0:00:22 [ 365MiB/s] [======================================================>] 100%
BENCH URLLIB3:
   8GiB 0:00:13 [ 601MiB/s] [======================================================>] 100%
BENCH REQUESTS
   8GiB 0:00:14 [ 562MiB/s] [======================================================>] 100%
BENCH GO HTTP
   8GiB 0:00:25 [ 320MiB/s] [======================================================>] 100%
```

Which is... unexpected