Fix debug logging

Fix an issue where debug logging never occurred because it was guarded
incorrectly using isWarnEnabled().

Fixes gh-1780

Author: philwebb

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

@@ -66,7 +66,7 @@ protected final File getValidDocumentRoot() {
 		file = file != null ? file : getExplodedWarFileDocumentRoot();
 		// Or maybe there is a document root in a well-known location
 		file = file != null ? file : getCommonDocumentRoot();
-		if (file == null && this.logger.isWarnEnabled()) {
+		if (file == null && this.logger.isDebugEnabled()) {
 			this.logger.debug("None of the document roots "
 					+ Arrays.asList(COMMON_DOC_ROOTS)
 					+ " point to a directory and will be ignored.");