Added text/plain content type support to configuration since it is

returned by some controller methods.

src/main/webapp/WEB-INF/arsnova-servlet.xml

@@ -25,6 +25,9 @@
 	<bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
 		<property name="messageConverters">
 			<list>
+	            <bean class = "org.springframework.http.converter.StringHttpMessageConverter">
+	                <property name="supportedMediaTypes" value = "text/plain;charset=UTF-8" />
+	            </bean>
 				<bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter" />
 			</list>
 		</property>