Do not use deprecated `util.pump`, it's going away.

[ChALkeR]

util.pump was deprecated since nodejs v0.10, and there is an ongoing discussion on removing that in the next major version: nodejs/node#2531.

The line in question: test/example.js#L13:

util.pump(res, memStream);

should be probably replaced with

res.pipe(memStream);

I am not making a pull request because I am not a user of this package and have not tested it.