Fixex generating Atom feed when adding description as summary

This patch fixes the problem that generating an Atom feed fails when
adding an RSS description marked as summary. The problem was that
feedgen internally expected a dictionary, but the value was stored as a
string in this case.

This fixes #94

feedgen/entry.py

@@ -499,7 +499,7 @@ def description(self, description=None, isSummary=False):
         if description is not None:
             self.__rss_description = description
             if isSummary:
-                self.__atom_summary = description
+                self.__atom_summary = {'summary': description}
             else:
                 self.__atom_content = {'content': description}
         return self.__rss_description