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

[baishuo]

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"

[SparkQA]

Test build #34409 has finished for PR 6695 at commit 3d19ad9.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #34420 has finished for PR 6695 at commit b742e69.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #34442 has finished for PR 6695 at commit 4df48c7.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #34444 has finished for PR 6695 at commit cb9f027.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[baishuo]

hi @yhuai would you please help me review this pr when you have time? i think maybe it was the base of #6494 . thanks:)

[marmbrus]

Remove the TODO.

[SparkQA]

Test build #34821 has finished for PR 6695 at commit 9e155f9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[baishuo]

hi @marmbrus ,thanks for review this PR. Had modified it according your comment. Would you please help to check whether it can be merged :)