Can't extend encoding by hack Buffer.isEncoding anymore

[hefangshi]

a widely used iconv library iconv-lite can extend encoding by hack Buffer.isEncoding using extendNodeEncodings

but a newly commit 8a94581 cached the Buffer.isEncoding method https://github.com/iojs/io.js/blob/master/lib/string_decoder.js#L3-L9 which made extendNodeEncodings won't work.

[Fishrock123]

@mscdex Looks like that could be undone easily enough without any real downsides. You on it?

[mscdex]

Fixed in 0fa6c4a.