fix converter issue by changing array to string with a foreach

[ghezelbash]

Convertor method is an array of methods in yireo but the rosell-dk/webp-convert module needs just one method as a string to do the convert. The convertors item is removed from $options array and a foreach added based on convertors object to send convertor methods one by one.

[jissereitsma]

Sorry for the long delay, but this is actually a pretty cool solid fix. It will be included in the next release! Thanks!