fix putStream for streams2

before this change, I have to do an additional pipe on the stream as a workaround:

https://github.com/superjoe30/node-multiparty/blob/master/examples/s3.js#L65

after this change everything works as expected with node v0.10 with putStream.

as a side note, multiparty has an example of streaming file uploads straight to s3 with knox without ever touching your hard drive. Hurray!