Fix ImageInterface encode() method argument

[edgarsn]

ImageInterface encode argument is not aligned with Image class.

ImageInterface.php

public function encode(EncoderInterface $encoder): EncodedImageInterface;

Image.php

public function encode(EncoderInterface $encoder = new AutoEncoder()): EncodedImageInterface

This is causing static analysis tools to fail as well as confuse IDEs with error:

Method Intervention\Image\Interfaces\ImageInterface::encode() invoked with 0 parameters, 1 required.

[olivervogel]

Thank you.