IllegalStateException in Tomcat's log after each use of 'Download immediately'

[Nicklas Nordborg]

Almost all export plug-ins have an option to download the result immediately to the client computer. There is no problem with the export but a log statement is created in one of the Tomcat log files.

java.lang.IllegalStateException: getOutputStream() has already been called for this response
	at org.apache.catalina.connector.Response.getWriter(Response.java:639)
	at org.apache.catalina.connector.ResponseFacade.getWriter(ResponseFacade.java:214)
	at org.apache.jasper.runtime.JspWriterImpl.initOut(JspWriterImpl.java:125)
	at org.apache.jasper.runtime.JspWriterImpl.flushBuffer(JspWriterImpl.java:118)
	at org.apache.jasper.runtime.PageContextImpl.release(PageContextImpl.java:190)
	at org.apache.jasper.runtime.JspFactoryImpl.internalReleasePageContext(JspFactoryImpl.java:126)
	at org.apache.jasper.runtime.JspFactoryImpl.releasePageContext(JspFactoryImpl.java:80)
	at org.apache.jsp.common.plugin.index_jsp._jspService(index_jsp.java:712)

[Nicklas Nordborg]

(In [6274]) Fixes #1755: IllegalStateException in Tomcat's log after each use of 'Download immediately'

Added call to clear the buffer before starting the export.