withPivot with `'!='` as operator would throw 'Illegal operator` error

[UnrulyNatives]

As described in detail here:
http://stackoverflow.com/questions/36856090/counting-models-related-through-manytomany-only-if-a-pivot-attribute-null-lara

The wherePivot would not accept '!=' operator.

This code produces a collection:

`public function activities()
{

return $this->belongsToMany('App\Models\Activity')->withPivot('done_at')->wherePivot('done_at','=',null);
}`

while this

`public function activitiesOnlyDone()
{

return $this->belongsToMany('App\Models\Activity')->withPivot('done_at')->wherePivot('done_at','!=',null);
}`

throws Illegal operator error.