fix race condition in decode-stream and add test

[bo-tato]

adds race condition test from #53
and a fix by appending an empty vec to src at the beginning rather than an on-drained callback to write an empty vec

[KingMob]

It's passing the tests consistently, I think you may have hit upon the solution. Wish I'd thought of using concat. Thanks!

[KingMob]

Fixes #60.

[KingMob]

Hey @bo-tato , how do you want to be thanked in the changelog? If you give me your name, I'll put it in there, or if you prefer, I can just use "bo-tato".

[bo-tato]

cool, can just use bo-tato