Minor fix: made "EXPLAIN" output to play well with JDBC output format

Fixed the broken JDBC output. Test from Shark `beeline`:

```
beeline> !connect jdbc:hive2://localhost:10000/  
scan complete in 2ms
Connecting to jdbc:hive2://localhost:10000/
Enter username for jdbc:hive2://localhost:10000/: lian
Enter password for jdbc:hive2://localhost:10000/: 
Connected to: Hive (version 0.12.0)
Driver: Hive (version 0.12.0)
Transaction isolation: TRANSACTION_REPEATABLE_READ
0: jdbc:hive2://localhost:10000/> 
0: jdbc:hive2://localhost:10000/> explain select * from src;            
+-------------------------------------------------------------------------------+
|                                     plan                                      |
+-------------------------------------------------------------------------------+
| ExplainCommand [plan#2:0]                                                     |
|  HiveTableScan [key#0,value#1], (MetastoreRelation default, src, None), None  |
+-------------------------------------------------------------------------------+
2 rows selected (1.386 seconds)
```