Fixed issue where non-existent included attributes are refetched upon access.

Where a find has include_fields define some attributes, where
those attributes are not defined in the obtained bugzilla
issue. An access to those fields will result in additional
xmlrpc searches.  Fix is to make sure those are defined as nil
in the returned bug hashes
Merge pull request #38 from abellotti/remove_unneccesary_xmlrpc_calls

Fixed issue where non-existent included attributes are refetched upon ac...