Update PMMLTranslator.java
git merge from upstream/hotfix-kevin-bugs
map missing/default to empty string zscl_value for string variables
Fix an Error that map default value to empty string bin value

In PMML, missing value could be considered as "empty string". But
default value is not, it means other candidiate values that could
not be found in the bin info table. In this case, default_value should
be set to "0.0".
fix bugs and eror predict for variable selection
Merge branch 'hotfix-kevin-bugs' of https://github.com/zhangjiaqi/shifu into zhangjiaqi-hotfix-kevin-bugs
fix the code from pull request