#8: NPE workaround

Author: vzhn

src/main/java/me/vzhilin/auth/digester/Ha1.java

@@ -55,7 +55,7 @@ public static Ha1 preHashed(DigestAlgorithm algorithm, String username, String r
     public static Ha1 hash(DigestAlgorithm algorithm, String username, String realm, String password) {
         MessageDigest instance;
         try {
-            if (algorithm == DigestAlgorithm.MD5 || algorithm == DigestAlgorithm.MD5_SESS) {
+            if (algorithm == DigestAlgorithm.MD5 || algorithm == DigestAlgorithm.MD5_SESS || algorithm == null) {
                 instance = MessageDigest.getInstance("MD5");
             } else
             if (algorithm == DigestAlgorithm.SHA_256 || algorithm == DigestAlgorithm.SHA_256_SESS) {