string_decoder: don't cache Buffer.isEncoding

Some modules are monkey-patching Buffer.isEncoding, so without this
they cannot do that.

Fixes: https://github.com/iojs/io.js/issues/1547
PR-URL: https://github.com/iojs/io.js/pull/1548


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>