Handle stream payload errors before socket connection

[ggoodman]

#223 introduced a small window between the creation of the request and the socket connection wherein error events on the payload would have no attached handler and could cause uncaught exceptions.

This PR includes a regression test as well as the fix:

• Attach a temporary error handler to the options.payload whose piping has been deferred
• When such error events happen, re-emit the error on the req itself and allow this to clean itself up

Open questions:

• should we remove the socket listener in onStreamError?
• should we even be using req.emit('error', err) as the mechanism to propagate the error?
• will things be correctly cleaned up for errors so-emitted?

[lock]

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