[SPARK-12136][STREAMING] rddToFileName does not properly handle prefix and suffix parameters

The original code does not properly handle the cases where the prefix is null, but suffix is not null - the suffix should be used but is not.

The fix is using StringBuilder to construct the proper file name.

Author: bomeng <bmeng@us.ibm.com>
Author: Bo Meng <mengbo@bos-macbook-pro.usca.ibm.com>

Closes #10185 from bomeng/SPARK-12136.

streaming/src/main/scala/org/apache/spark/streaming/StreamingContext.scala

@@ -892,12 +892,13 @@ object StreamingContext extends Logging {
   }
 
   private[streaming] def rddToFileName[T](prefix: String, suffix: String, time: Time): String = {
-    if (prefix == null) {
-      time.milliseconds.toString
-    } else if (suffix == null || suffix.length ==0) {
-      prefix + "-" + time.milliseconds
-    } else {
-      prefix + "-" + time.milliseconds + "." + suffix
+    var result = time.milliseconds.toString
+    if (prefix != null && prefix.length > 0) {
+      result = s"$prefix-$result"
+    }
+    if (suffix != null && suffix.length > 0) {
+      result = s"$result.$suffix"
     }
+    result
   }
 }