ftp: fix Put mkParentDir failed: 521 for BunnyCDN - fixes #2363

According to RFC 959, error 521 is the correct error return to mean
"dir already exists", so add support for this.

backend/ftp/ftp.go

@@ -480,6 +480,8 @@ func (f *Fs) mkdir(abspath string) error {
 		switch errX.Code {
 		case ftp.StatusFileUnavailable: // dir already exists: see issue #2181
 			err = nil
+		case 521: // dir already exists: error number according to RFC 959: issue #2363
+			err = nil
 		}
 	}
 	return err