Changes to enable images of type 300×0

To declare a size where one side is calculated by the other, usually one can set the variable of the calculated side with 0, like this

add_image_size( 'keep-size', 310, 0, true );

To allow these type of sizes in this plugin, change the line 155 of fly-dynamic-image-resizer.php with

if ( '' == $size_name || ( !$width && !$height ) ) {