Return Buffer objects from requests where it makes sense

See: http://stackoverflow.com/questions/30869460/node-js-browserify-error-on-parsing-tar-file/30878794#30878794, https://github.com/substack/stream-browserify/issues/8

It would be nice if this module emitted actual instances of Buffer so that it is compatible with other streams, specifically for `.pipe`

Start by tracing the request and response stream implementation to find where data chunks are emitted, then review the linked Stack Overflow and stream-browserify discussions. Confirm which request paths should emit Buffer instances for .pipe compatibility; done means those paths consistently return compatible Buffer objects and existing stream behavior remains intact.

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