[MRG+1] Added unit test for adding classes_ property to GridSearchCV, fixes #6298 (#7661)

* Fix issue #6298
Adds a "classes_" property to BaseSearchCV

* Added test to ensure classes_ property is added to gridSearch correctly

* Fixed formatting

* Added test to ensure gridSearchCV with a regressor does not have a classes_ attribute

* Fixed whitespace issues

* Combined tests for the new GridSearchSV.classes_ property into a single test.

* Removed trailing whitespace

* Added what's new for pull request #7661

* Fixed formatting of update in what's new