HDDS-1087. Fix TestDefaultCertificateClient#testSignDataStream. Contributed by Xiaoyu Yao. (#596)

hadoop-hdds/common/src/test/java/org/apache/hadoop/hdds/security/x509/certificate/client/TestDefaultCertificateClient.java

@@ -152,7 +152,7 @@ public class TestDefaultCertificateClient {
 
   @Test
   public void testSignDataStream() throws Exception {
-    String data = RandomStringUtils.random(100);
+    String data = RandomStringUtils.random(100, UTF);
     // Expect error when there is no private key to sign.
     LambdaTestUtils.intercept(IOException.class, "Error while " +
             "signing the stream",