fix exception with smart detection + gif + meta endpoint
fix exception with smart detection + gif + meta endpoint

the JSONEngine was not having the `extension` attribute stored from the
wrapped Engine which leads to the exception fixed (ignored) in #841

with the added extension part the transformer won't call smart detection
if gifsicle is enabled and it's a GIF image

okor/thumbor,kkopachev/thumbor,marcelometal/thumbor,scorphus/thumbor,scorphus/thumbor,okor/thumbor,thumbor/thumbor,kkopachev/thumbor,marcelometal/thumbor,thumbor/thumbor,kkopachev/thumbor,thumbor/thumbor,gi11es/thumbor,marcelometal/thumbor,thumbor/thumbor,gi11es/thumbor,scorphus/thumbor,scorphus/thumbor,voxmedia/thumbor,okor/thumbor,voxmedia/thumbor,gi11es/thumbor,kkopachev/thumbor,voxmedia/thumbor