Support for WebWorkers

[jayfunk]

stream-http assumes a normal browser window and not a worker, though http requests in a worker are legitimate.

Inside a webworker the global object is self not window.

Original issue in http-browserify.

[jhiesey]

Hm, it's very easy to just replace window with global, but I wonder if there's anything else that is different between workers and the window.

I'll add a test for WebWorker support before merging that change.

[jhiesey]

Fixed by 8cab16f