IMPORTANT FIX: small strings weren't getting a proper UTF-8 encoding (xkihzew)

`(.getBytes <string>)` was being used here instead of
`(.getBytes <string> "UTF-8")` as is correct and done elsewhere.

This means that small UTF-8 strings may have been incorrectly stored
in environments where UTF-8 is not the default JVM character encoding.

Bug was introduced in Nippy v2.6.0, released 9 days ago (2014 Mar 30).

*********************************************************************
Please check for possible errors in Unicode text written using Nippy
v2.6.0 if your JVM uses an alternative character encoding by default
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Really sorry about this, not sure how this slipped in.

Thanks to @xkihzew for the bug report.
v2.6.1