FIX: update functions are lost during cloning of a design doc

To reproduce the issue we need only a design doc with one or several update functions. When we run the following command on such a ddoc, the update functions are lost.
```
couchapp clone http://127.0.0.1:5984/testdb/_design/helloworld helloworld
```
Due to official docs 'updates' but not 'update' should be used as a directory name:
http://wiki.apache.org/couchdb/Document_Update_Handlers