Not serializable exception during computeFeatureStats

[alkersan]

Description
Serializability issue during one of the stages of model training:

... 
(TID 3067704) had a not serializable result: scala.collection.immutable.MapLike$$anon$2
Serialization stack:
    - object not serializable (class: scala.collection.immutable.MapLike$$anon$2)
...

Stacktrace leads to com.salesforce.op.filters.PreparedFeatures#summaries method, which is called from com.salesforce.op.filters.RawFeatureFilter#computeFeatureStats. The usage
of mapValues leads to a non serializable Map, which is a long standing scala bug.

I've attempted to fix that just by adding a .map(identity), which resolved the issue, i.e.:

predictors.mapValues(Summary(_)).map(identity)

Not sure about "correct" way of dealing with that

Additional context
Transmogrify version: 0.5.1 & 0.5.2. Though it seems to be applicable to current master, as the related code didn't change since then.

[tovbinm]

Thanks for reporting this. Fixed in #363