[SPARK-8156] [SQL] create table to specific database by 'use dbname'

when i test the following code:
hiveContext.sql("""use testdb""")
val df = (1 to 3).map(i => (i, s"val_$i", i * 2)).toDF("a", "b", "c")
df.write
.format("parquet")
.mode(SaveMode.Overwrite)
.saveAsTable("ttt3")
hiveContext.sql("show TABLES in default")

found that the table ttt3 will be created under the database "default"

Author: baishuo <vc_java@hotmail.com>

Closes #6695 from baishuo/SPARK-8516-use-database and squashes the following commits:

9e155f9 [baishuo] remove no use comment
cb9f027 [baishuo] modify testcase
00a7a2d [baishuo] modify testcase
4df48c7 [baishuo] modify testcase
b742e69 [baishuo] modify testcase
3d19ad9 [baishuo] create table to specific database