Fix files_external webdav up and download when path contains ' '

I wanted to mount a webdav share in owncloud and stumbled over this. listing files works fine because it is done with sabredav. fopening a file is done with curl and will fail when the path contains ' '. this PR replaces it with %20 for fopen and upload via curl. 

@MTGap @icewind1991 @DeepDiver1975 please review