stream: avoid unnecessary concat of a single buffer.
stream: avoid unnecessary concat of a single buffer.

Avoids doing a buffer.concat on the internal buffer
when that array has only a single thing in it.

Reviewed-By: Chris Dickinson <711c73f64afdce07b7e38039a96d2224209e9a6c@neversaw.us>
Reviewed-By: James M Snell <2d64a127ffc94dbe6295928a16bedf8b26814159@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/3300

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node