[SPARK-5002][SQL] Using ascending by default when not specify order in order by

[scwf]

spark sql does not support SELECT a, b FROM testData2 ORDER BY a desc, b.

[SparkQA]

Test build #24886 has started for PR 3838 at commit 114b64a.

• This patch merges cleanly.

[SparkQA]

Test build #24886 has finished for PR 3838 at commit 114b64a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • class GaussianMixtureModel(

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/24886/
Test PASSed.

[marmbrus]

Thanks, merged to master.