Make DEFAULT_PROTOCOL public

Fixes gh-2110

Author: philwebb

spring-boot/src/main/java/org/springframework/boot/context/embedded/tomcat/TomcatEmbeddedServletContainerFactory.java

@@ -82,7 +82,7 @@
 public class TomcatEmbeddedServletContainerFactory extends
 		AbstractEmbeddedServletContainerFactory implements ResourceLoaderAware {
 
-	private static final String DEFAULT_PROTOCOL = "org.apache.coyote.http11.Http11NioProtocol";
+	public static final String DEFAULT_PROTOCOL = "org.apache.coyote.http11.Http11NioProtocol";
 
 	private File baseDirectory;