[SPARK-20043][ML] DecisionTreeModel: ImpurityCalculator builder fails for uppercase impurity type Gini

Fix bug: DecisionTreeModel can't recongnize Impurity "Gini" when loading

TODO:
+ [x] add unit test
+ [x] fix the bug

Author: 颜发才（Yan Facai） <facai.yan@gmail.com>

Closes #17407 from facaiy/BUG/decision_tree_loader_failer_with_Gini_impurity.

(cherry picked from commit 7d432af8)
Signed-off-by: Joseph K. Bradley <joseph@databricks.com>