[SPARK-4847][SQL]Fix "extraStrategies cannot take effect in SQLContext" issue

Author: jerryshao <saisai.shao@intel.com>

Closes #3698 from jerryshao/SPARK-4847 and squashes the following commits:

4741130 [jerryshao] Make later added extraStrategies effect when calling strategies

(cherry picked from commit dc8280dcca7b54793a3db644f74fd33460960d4a)
Signed-off-by: Michael Armbrust <michael@databricks.com>