Emit data event data as a Buffer

[deathcap]

For piping workerstreams to other streams (I'm using a websocket-stream) it is convenient for the data event payload to be of type Buffer, instead of Uint8Array. This PR checks whether the event data is a typed array, and if so converts it to a (browserify) buffer using typedarray-to-buffer. browserify-buffers are simply augmented Uint8Arrays, so typedarray-to-buffer is a zero-copy operation.

websocket-stream also performs a Buffer conversion, here: max-mapper/websocket-stream@0089ece#diff-168726dbe96b3ce427e7fedce31bb0bcR58 (as of 1.0.0) - so I think it makes sense for workerstream to do so as well (with pre-1.0 versions of websocket-stream, I ran into the same problem being fixed here in workerstream)

[deathcap]

Not 100% sure this is the right approach, made this also: https://github.com/deathcap/tobuffer-stream can pipe as follows (works with workerstream 1.2.1): self.parentStream.pipe(toBufferStream)