[encoding] What's the correct choice for BIG5 F9FE? (#28)

According to:
https://lists.w3.org/Archives/Public/public-whatwg-archive/2012Apr/0095.html
The decoding for F9FE from different vendor are different,
And the `whatwg encoding` standard are choose
`F9FE ` -> `U+FFED`, is that the correct behavior 
```
> F9FE =>
> opera-hk: U+FFED
> firefox: U+2593
> chrome: U+2593
> firefox-hk: U+2593
> opera: U+2593
> chrome-hk: U+FFED
> internetexplorer: U+2593
> windows-os: U+2593
> hkscs-2008: <U+FFED>
```

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/encoding/issues/28