Middleware Handler

Notice: This version drops node < 4.0 support.

Fixed

• Correct the Cookie header usage to remove possible duplicates (28eb9e3)
• Improved the usage of handling the end of the middleware chain to invoke the transport handler - previously it always created a promise instance even though it may never have resolved (e.g. in the case of cached responses, it would not call next())

Changed

• Update to TypeScript 2.1
• Shrunk dependency footprint (extend and promises are available in node >= 4)
• Refactor progress and abort to use an internal event system which is propagated to cloned instances