fixes #1405
if view argument is not passed to config.add_notfound_view, use default_exceptionresponse_view
properly detect undefined view
if view argument is not passed to config.add_forbidden_view, use default_exceptionresponse_view
Merge pull request #1446 from iElectric/guess_mimtype_test_fix

fixes #1405
update changelog
Merge pull request #1447 from iElectric/add_notfound_view/default_exceptionresponse_view

Add notfound view/default exceptionresponse view
update changelog