[5.4] Set connection while creating a model

[themsaid]

This PR adds the ability to set the desired connection to create the model on.

To save a model on a different connection you currently need to do something like:

$model = new User();

$model->setConnection('myConnection');

$model->save();

With this PR, by default we create the model on the default model connection, only if a connection is provided to the methods we'll use it to create our model.

[themsaid]

@taylorotwell there's already a way to create models on a different connection as the above example, so feel free to close the PR if you think adding the connection argument adds some complexity to the code.

[taylorotwell]

Thanks