[5.6] Add tests for updating existing pivot + add option for passing model

[bobbybouwmann]

Added the parseId method to the InteractsWithPivotTable trait. This way we can now use models to update existing pivot values. For example

// Before (which still works as well)
$post->tags()->updateExistingPivot($tag->id, ['key' => 'value']);

// After
$post->tags()->updateExistingPivot($tag, ['key' => 'value']);

In addition to that I also added the missings tests for this method ;)

If for some reason you decide not to take the model part, you can at least add the tests 👍