ResponseWriter is not working with composite Accept headers

[agazzarini]

e.g. Accept: Accept: text/turtle; charset=utf-8

produces:

16:33:22,296 INFO [store] webapp=/solr path=/rdf-graph-store params={default=} status=0 QTime=1063
16:33:22,309 ERROR null:org.apache.jena.riot.RiotException: No graph writer for null
at org.apache.jena.riot.RDFDataMgr.createGraphWriter$(RDFDataMgr.java:1304)
at org.apache.jena.riot.RDFDataMgr.write$(RDFDataMgr.java:1318)
at org.apache.jena.riot.RDFDataMgr.write(RDFDataMgr.java:1085)
at org.apache.jena.riot.RDFDataMgr.write(RDFDataMgr.java:1075)
at org.apache.jena.riot.RDFDataMgr.write(RDFDataMgr.java:1009)
at org.gazzax.labs.solrdf.response.SPARQLResponseWriter$3.doWrite(SPARQLResponseWriter.java:139)
at org.gazzax.labs.solrdf.response.SPARQLResponseWriter.write(SPARQLResponseWriter.java:72)
at org.apache.solr.servlet.SolrDispatchFilter.writeResponse(SolrDispatchFilter.java:762)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:431)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:207)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1288)