@last_modified should ignore lack of etag method when USE_ETAGS is enabled

[Chris Lamb]

Using @last_modified with settings.USE_ETAGS does not work as expected.

My goal is to avoid evaluating the view for a select handful of expensive pages based on the Last-Modified header, and globally fallback to saving bandwidth with USE_ETAGS.

However, if you enable USE_ETAGS and the @last_modified decorator, the view is still evaluated despite the last_modified_func being a match with If-Modified-Since. This is because the client is providing an ETag header (added because USE_ETAGS is enabled) and django.views.decorators.http.condition uses that etag along with the fact that we did did not provide an etag_func argument as a reason for not returning 304.

Patch attached that disables this particular part of the logic if settings.USE_ETAGS is enabled.

[Chris Lamb]

duplicate, sorry.