string_decoder: don't cache Buffer.isEncoding
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 <ad0310dc5270cefb6ee6879263270aed9b1e9982@me.com>
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>

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