Can't print a Mapper object

[JordanP]

While fixing #11 a bug was introduced.

table = [('Route name', 'Methods', 'Path', 'Controller', 'action')] + \
        [(r.name or '', format_methods(r), r.routepath or '',
          r.defaults.get('controller', ''), r.defaults.get('action', ''))
         for r in self.matchlist]

widths = [max(len(row[col]) for row in table)
          for col in range(len(table[0]))]

While iterating over all the rows and all the columns of the table, a call to len(r.defaults.get('controller', '')) is going to be made. But r.defaults.get('controller', '') is an instance of a class that doesn't have to have a __len__ method. So this code will fail with TypeError: object of type 'ResourceV21' has no len().