[SQL] SPARK-1922

Allow underscore in column name of a struct field https://issues.apache.org/jira/browse/SPARK-1922 .

Author: LY Lai <ly.lai@vpon.com>

Closes #873 from lyuanlai/master and squashes the following commits:

2253263 [LY Lai] Allow underscore in struct field column name

(cherry picked from commit 068256745052b0aa947dd8c16b1f1d73d8e4631e)
Signed-off-by: Reynold Xin <rxin@apache.org>