'scan_api_tree' adds each route once

Alter 'scan_api_tree' function to only call 'add_route' for each
complete route once.
'default_view_kwargs' tests

Add tests for 'default_view_kwargs' class attribute. This value
serves as defaults for the 'view_kwargs' dict of a view callable
instance.
'default_view_kwargs'

Implement 'default_view_kwargs' view callable class attribute.