WebDavSqlStoreCollectionFactory's Enabled parameter is not honored

The Enabled parameter in WebDavSqlStoreCollectionFactory is set but never used.

The fix is to add at the beginning of method GetCollection (line 41) the following lines:

```
if (!Enabled)
    return new WebDavSqlStoreCollection(parentCollection, path, rootPath, rootGuid, Store);
```

This allows better troubleshooting of any issue during testing.