Fix process leak causing accumulation of zombie Ruby processes

Hi! Don't know if you're interested in this pull request, since your repo is so old, but I fixed a fairly subtle but major concurrency bug in rack-streaming-proxy; thought you might like to have it.

The details on the bug is here:

http://siliconisland.ca/2013/04/26/beware-of-the-zombie-process-apocalypse/