[5.5] Add get method in the Relation class

[sileence]

(Reopened)

This PR is a micro optimisation on one side, but on the other side I think it is good to be explicit here since the BelongsToManyclass is actually "overriding" the (non-declared) get method.

For example I thought I could change this:

    public function getEager()
    {
        return $this->get();
    }

To this:

    public function getEager()
    {
        return $this->query->get();
    }

Since get()was just a call to a "magic method"

But this actually caused 2 tests to fail because get is (re)declared in BelongsToMany.