Make ErrorPageFilter public

Change the visibility of ErrorPageFilter to public to fix
IllegalAccessException errors on certain servlet containers.

Fixes gh-2026

Author: philwebb

spring-boot/src/main/java/org/springframework/boot/context/web/ErrorPageFilter.java

@@ -56,8 +56,8 @@
  */
 @Component
 @Order(Ordered.HIGHEST_PRECEDENCE)
-class ErrorPageFilter extends AbstractConfigurableEmbeddedServletContainer implements
-		Filter, NonEmbeddedServletContainerFactory {
+public class ErrorPageFilter extends AbstractConfigurableEmbeddedServletContainer
+		implements Filter, NonEmbeddedServletContainerFactory {
 
 	private static Log logger = LogFactory.getLog(ErrorPageFilter.class);