fix "set MASTER automatically fails" bug.

spark-shell intends to set MASTER automatically if we do not provide the option when we start the shell , but there's a problem. 
The condition is "if [[ "x" != "x$SPARK_MASTER_IP" && "y" != "y$SPARK_MASTER_PORT" ]];" we sure will set SPARK_MASTER_IP explicitly, the SPARK_MASTER_PORT option, however, we probably do not set just using spark default port 7077. So if we do not set SPARK_MASTER_PORT, the condition will never be true. We should just use default port if users do not set port explicitly I think.

bin/spark-shell

@@ -45,13 +45,18 @@ for o in "$@"; do
 done
 
 # Set MASTER from spark-env if possible
+DEFAULT_SPARK_MASTER_PORT=7077
 if [ -z "$MASTER" ]; then
   if [ -e "$FWDIR/conf/spark-env.sh" ]; then
     . "$FWDIR/conf/spark-env.sh"
   fi
-  if [[ "x" != "x$SPARK_MASTER_IP" && "y" != "y$SPARK_MASTER_PORT" ]]; then
-    MASTER="spark://${SPARK_MASTER_IP}:${SPARK_MASTER_PORT}"
-    export MASTER
+  if [ "x" != "x$SPARK_MASTER_IP" ];then
+    if [ "y" != "y$SPARK_MASTER_PORT" ];then
+      SPARK_MASTER_PORT="${SPARK_MASTER_PORT}"
+    else
+      SPARK_MASTER_PORT=$DEFAULT_SPARK_MASTER_PORT
+    fi
+    export MASTER="spark://${SPARK_MASTER_IP}:${SPARK_MASTER_PORT}"
   fi
 fi