Allow encryption method none.

lib/zip_file_set_encryption.c

diff --git a/lib/zip_file_set_encryption.c b/lib/zip_file_set_encryption.c
index b9312b4..e604e6f 100644
--- a/lib/zip_file_set_encryption.c
+++ b/lib/zip_file_set_encryption.c
@@ -53,7 +53,7 @@
 	return -1;
     }
 
-    if (_zip_get_encryption_implementation(method, ZIP_CODEC_ENCODE) == NULL) {
+    if (method != ZIP_EM_NONE && _zip_get_encryption_implementation(method, ZIP_CODEC_ENCODE) == NULL) {
 	zip_error_set(&za->error, ZIP_ER_ENCRNOTSUPP, 0);
 	return -1;
     }