Fix Crossvalidation when dataframe is not provided

[americast]

The error arising when a dataframe is not passed when cross_validation is called is addressed in this PR. Fixes #800.

Looking forward to reviews and suggestions.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}