Properly format Content-Length: header on 32-bit systems.

- Promote len argument in dump_content_length to uint64_t.
- Make sure there is sufficient scratch space to format string.
- Use PRIu64 macro for formatting.

Signed-off-by: Jan Harkes <jaharkes@cs.cmu.edu>
Merge pull request #92 from jaharkes/content_length

Properly format Content-Length: header on 32-bit systems.

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>