[SPARK-8051] [MLLIB] make StringIndexerModel silent if input column does not exist

This is just a workaround to a bigger problem. Some pipeline stages may not be effective during prediction, and they should not complain about missing required columns, e.g. `StringIndexerModel`. jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #6595 from mengxr/SPARK-8051 and squashes the following commits:

b6a36b9 [Xiangrui Meng] add doc
f143fd4 [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-8051
8ee7c7e [Xiangrui Meng] use SparkFunSuite
e112394 [Xiangrui Meng] make StringIndexerModel silent if input column does not exist