Alter jvm_scala_object to better handle nested scala classes.

[jsleight]

The enums for binary operation (and others) are implemented as nested case objects, which means they have class paths like the following:

$ ls mleap-core/target/scala-2.12/classes/ml/combust/mleap/core/feature/ | grep BinaryOperation
BinaryOperation$.class
BinaryOperation$Add$.class
BinaryOperation$Divide$.class
BinaryOperation$LogN$.class
BinaryOperation$Max$.class
BinaryOperation$Min$.class
BinaryOperation$Multiply$.class
BinaryOperation$Pow$.class
BinaryOperation$Remainder$.class
BinaryOperation$Subtract$.class
BinaryOperation.class

This PR makes it so that we'll search in the jvm for ml.combust.mleap.core.feature.BinaryOperation$Add$ instead of the current ml.combust.mleap.core.feature.BinaryOperation.Add$. To be honest I'm not sure why the current code doesn't break the existing tests.

Release Note

I have this PR based off of the v0.21.0 tag and intend to release this as v0.21.1. I'll also make a PR to apply this commit to master branch (which will later be released as v0.22.0)

[austinzh]

LGTM.
We should create a release branch first thought