buffer: default to UTF8 in byteLength()

If an undefined encoding is passed to byteLength(),
assume that it is UTF8 immediately. This yields a
small speedup, as it prevents string operations on
the encoding argument.

PR-URL: https://github.com/nodejs/node/pull/4010


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>