Fix for exception in chart export with Chrome

The entity "nbsp" was referenced, but not declared.
	at org.apache.batik.transcoder.XMLAbstractTranscoder.transcode(XMLAbstractTranscoder.java:136)
	at org.apache.batik.transcoder.SVGAbstractTranscoder.transcode(SVGAbstractTranscoder.java:156)
	at org.saiku.web.svg.BatikConverter.convert(Converter.java:86)
	at org.saiku.web.rest.resources.ExporterResource.exportChart(ExporterResource.java:166)
	at org.saiku.web.rest.resources.ExporterResource$$FastClassByCGLIB$$e2189a5.invoke(<generated>)
Merge pull request #218 from lduburas/bug

Fix for exception in chart export with Chrome