[MRG+1] Support of the collections.Sequence type has been added to the...

[MRG+1] Support of the collections.Sequence type has been added to the _check_param_grid method from model_selection. (#7323)

* Support of the collections.Sequence type has been added to the _check_param_grid method from model_selection.

* test_grid_search_when_param_grid_includes_range test was refactored (parts that are not nessesary have been removed).

* test_grid_search_bad_param_grid now checks that value is not string. This is important since string is a Sequence.

* _check_param_grid now checks is the type is not string together with the check for other types.

* whats_new.rst has been updated to include information about bug fix for bug #7322.

* Description of the fix for the bug #7322 has been updated.

* Fix for indented in model_selection._search.py.