string_decoder: optimize write()

By limiting property getting/setting to only where they are
absolutely necessary, we can achieve greater performance
especially with small utf8 inputs and any size base64 inputs.

PR-URL: https://github.com/iojs/io.js/pull/1209


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Nicu Micleușanu <micnic90@gmail.com>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>