Django 1.11 middleware compatibility fix (#42)

* Use MiddlewareMixin for compatibility.

* Handle ImportError if Django version < 1.11.

* Remove var being overwritten by subsequent boolean expression.

* Update documentation to mention MIDDLEWARE and MIDDLEWARE_CLASSES may be used.

docs/usage.rst

@@ -20,11 +20,12 @@ Make sure the module is installed as an app:
         'hawkrest',
     )
 
-Make sure the middleware is installed:
+Make sure the middleware is installed by adding it to your project's
+``MIDDLEWARE`` or ``MIDDLEWARE_CLASSES`` (Django version < 1.11) setting:
 
 .. code-block:: python
 
-    MIDDLEWARE_CLASSES = (
+    MIDDLEWARE = (
         ...
         'hawkrest.middleware.HawkResponseMiddleware',
     )

hawkrest/middleware.py

@@ -1,13 +1,19 @@
 import logging
 
+try:
+    from django.utils.deprecation import MiddlewareMixin
+    middleware_cls = MiddlewareMixin
+except ImportError:  # Django version < 1.11
+    middleware_cls = object
+
+
 log = logging.getLogger(__name__)
 
 
-class HawkResponseMiddleware:
+class HawkResponseMiddleware(middleware_cls):
 
     def process_response(self, request, response):
         is_hawk_request = False
-        hawk_auth_was_processed = False
         if request.META.get('HTTP_AUTHORIZATION', '').startswith('Hawk'):
             is_hawk_request = True