Previously, if the user tried to submit content which was
malformed utf-8, Instiki would complain loudly to him.
A slightly more user-friendly approach was suggested by
the latest Rails 2.3.4, and a conversation with Sam Ruby
(who suggested some improvements).
Now, instead of complaining, we remove the offending bytes,
leaving a well-formed utf-8 string, which we pretend is what
the user meant to submit.
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