Adding fix for transparent pngs

[rmaglio]

The Pillow library was changed where it throws an error when you save a transparent PNG as a JPG because there was no place for the transparent channel to be mapped to in a jpg so the transparent channel is dropped. To avoid the error you need to explicitly convert the PNG to use RGB and then save it.

[coveralls]

Coverage remained the same at 92.857% when pulling dd86ced on rmaglio:master into e8b1679 on yu-liang-kono:master.

[shoeb-a-sabur]

Is the repo maintainer active? Merging this would have saved me a few hours of searching and trying stuff. Although I solved it a bit differently by doing a shallow copy and adding a check to make the conversion only when PNG with alpha channel is detected

`

    working_image = copy.copy(self.context.modules.engine.image)

    if working_image.mode in ('RGBA', 'LA'):
        logging.debug("RGBA found and converting")
        working_image = working_image.convert("RGB")

    bio = BytesIO()
    working_image.save(bio, 'JPEG')
    return bio.getvalue()

`