Add changeRate method to Reaction contract

[antonkomarev]

Motivation

Reacter::reactTo method changes reaction rate directly using Eloquent update method:

$reaction->update([
    'rate' => $rate,
]);

But method update does not exist on Cog\Contracts\Love\Reaction\Models\Reaction interface.

New Implementation

We can add changeRate method to the Reaction interface.

public function changeRate(float $rate): void;

We could move check for same rate value out from the Reacter::reactTo method to new Reaction::changeRate method.

if ($reaction->getRate() === $rate) {
    throw new ReactionAlreadyExists(
        sprintf('Reaction of type `%s` with `%s` rate already exists.', $reactionType->getName(), $rate)
    );
}

This will require to introduce new exception like RateInvalid.