Became a maintainer of rack-streaming-proxy

Hi, @fredngo. 

As I issued at the original repository of rack-streaming-proxy https://github.com/zerowidth/rack-streaming-proxy/issues/5, I became a maintainer of rack-streaming-proxy. 

After we pull requested to your repository https://github.com/fredngo/rack-streaming-proxy/pull/3, we fixed issues such as

1. Fix spec https://github.com/yohoushi/rack-streaming-proxy/pull/1
2. Do not response chunked body to HTTP/1.0 client https://github.com/yohoushi/rack-streaming-proxy/pull/2

The contents of this pull request is corresponding with the first one, but it does not matter. 

@zerowidth says

> Can you coordinate with @fredngo before pushing new releases, just to make sure you're not stepping on each other's toes?

I am planning to update the original repository by the contents of our repository, and release new gem. Is it fine?