ignore nullable in DeepImageFeaturizer.validateSchema

[mengxr]

In Spark SQL, nullability is a hint used during optimization and codegen to skip nullchecks, but not intended as an enforcement mechanism or as an implication that null values do exist. It might get dropped through the pipeline.

This PR switches to DataType.equalsIgnoreNullability for the check. Without the change, the test would fail with:

[info] - DeepImageFeaturizer accepts nullable *** FAILED ***
[info]   java.lang.ClassCastException: org.apache.spark.sql.types.StructField cannot be cast to org.apache.spark.sql.types.StructType
[info]   at com.databricks.sparkdl.DeepImageFeaturizerSuite$$anonfun$8.apply(DeepImageFeaturizerSuite.scala:134)
[info]   at com.databricks.sparkdl.DeepImageFeaturizerSuite$$anonfun$8.apply(DeepImageFeaturizerSuite.scala:132)
[info]   at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85)
[info]   at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
[info]   at org.scalatest.Transformer.apply(Transformer.scala:22)
[info]   at org.scalatest.Transformer.apply(Transformer.scala:20)
[info]   at org.scalatest.FunSuiteLike$$anon$1.apply(FunSuiteLike.scala:186)
[info]   at org.scalatest.TestSuite$class.withFixture(TestSuite.scala:196)
[info]   at org.scalatest.FunSuite.withFixture(FunSuite.scala:1560)
[info]   at org.scalatest.FunSuiteLike$class.invokeWithFixture$1(FunSuiteLike.scala:183)

cc: @jkbradley

[codecov-io]

Codecov Report

Merging #143 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   85.37%   85.37%   +<.01%     
==========================================
  Files          33       34       +1     
  Lines        1921     1922       +1     
  Branches       44       41       -3     
==========================================
+ Hits         1640     1641       +1     
  Misses        281      281

Impacted Files	Coverage Δ
...a/com/databricks/sparkdl/DeepImageFeaturizer.scala	93.65% <100%> (ø)	:arrow_up:
...cala/org/apache/spark/sql/types/DataTypeShim.scala	100% <100%> (ø)

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