Use UTF-8 encoding in FusekiExportService
Non-ASCII characters in metadata records were being replaced by U+FFFD REPLACEMENT CHARACTER. This is because the default encoding for non-ASCII characters is ISO-8859-1, rather than the modern standard of UTF-8. At some point in sending the request, it gets reinterpreted as (invalid) UTF-8 and the ISO-8859-1 characters are replaced.
We enforce the encoding as UTF-8 by changing the Content-Type to include the correct charset.
Closes EMC-148