[MRG+1] Accept keyword parameters to hyperparameter search fit methods (#8278)

* ENH Accept keyword parameters to hyperparameter search fit methods

Deprecate ``fit_params`` as a constructor argument to the hyperparameter search classes and instead accept keyword parameters to the ``fit`` methods. This makes the ``fit`` methods of these functions conform to the Estimator API and allows the use of hyperparameter search functions in other CV utility functions such as ``cross_val_predict``.

* CR: Expanded tests, remove deprecated use in Ridge

* Make tests consistent in Python 2 and 3