fix empty file input form submit (fixes #015)
fix empty file input form submit (fixes #015)

zamok2971/GoutteWithMultiCurl,viniciusferreira/Goutte,moffe42/Goutte,ferjflores/Goutte,chrismoulton/Goutte,sitesense/Goutte,thewilkybarkid/Goutte,farhan4648gul/Goutte,FriendsOfPHP/Goutte,2upmedia/Goutte,zamok2971/GoutteWithMultiCurl,trustable/Goutte,jozhao/Goutte,stof/Goutte,zamok2971/GoutteWithMultiCurl,georaldc/Goutte