Fix extractJIRA() function in Atlassian.groovy

Related-To: PRODX-3678
Change-Id: I622ab7ed220c2f838dc208abb5b5b9eb90da0a0e
diff --git a/src/com/mirantis/mk/Atlassian.groovy b/src/com/mirantis/mk/Atlassian.groovy
index ef28862..a3f08dc 100644
--- a/src/com/mirantis/mk/Atlassian.groovy
+++ b/src/com/mirantis/mk/Atlassian.groovy
@@ -46,7 +46,7 @@
 **/
 
 List extractJIRA(String commitMsg, String matcherRegex = '([A-Z]+-[0-9]+)') {
-  String msg = commitMsg.decodeBase64()
+  String msg = new String(commitMsg.decodeBase64())
   def matcher = (msg =~ matcherRegex)
   List tickets = []