Django's base View class for class-based views performs a check before

dispatching to make sure that the HTTP method in question is allowed,

returning a HTTP 405 if not. While useful, this is too late for more

complex views that are performing other (potentially method-specific)

logic before reaching View.dispatch.
This change introduces CheckRequestMethodViewMixin, which can be

placed at the beginning of the inheritance list to handle the checking

early. It will perform the same checks that View.dispatch performs,

but can be done before any other mixin dispatch methods are invoked.

Unit tests pass.
Made use of this in another change and verified the dispatch handlers

never got called for unsupported HTTP methods.