[SPARK-2615] [SQL] Add Equal Sign "==" Support for HiveQl

Currently, the "==" in HiveQL expression will cause exception thrown, this patch will fix it.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #1522 from chenghao-intel/equal and squashes the following commits:

f62a0ff [Cheng Hao] Add == Support for HiveQl

(cherry picked from commit 79fe7634f6817eb2443bc152c6790a4439721fda)
Signed-off-by: Michael Armbrust <michael@databricks.com>