#8: NPE workaround

Author: vzhn

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

@@ -133,7 +133,7 @@ private String h(String string, DigestAlgorithm algorithm) {
         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) {