[5.4] Add whereKey method

[mnabialek]

This method will make easier to add condition with primary key (what is not always id). It is also useful when you want to get one column only. For example instead of:

return ($model = Model::find(1)) ? $model->value('name') : null;

or

return $model->where('this_is_key_name',1)->value('name');

you can use:

return $model->whereKey(1)->value('name');

This is breaking change (all occurrences of whereKey( method in existing app should be changed to where('key', so it's in 5.4 as agreed in #16540