Enforce geoserver export is UTF-8.

[transistorize]

The version of Jetty we use defaults to responding with the ISO-88591 charset in the headers. All the database code uses UTF-8, and I don't think the charsets should change along the pipeline. I'm not a web expert, but it seems we should stick with one format in the ContentType headers. One way to fix this is to enforce the encoding in the response servlet. I'm concerned we have bugs in the roundtrip between config, import, and export api endpoints. Need to investigate the ramifications of this on client code.