Avoid using deprecated Django request.is_ajax() (#160)

Django 3.1 deprecated the `is_ajax` method on `HTTPRequest` and added a `RemovedInDjango40Warning`. The [release notes state](https://docs.djangoproject.com/en/3.1/releases/3.1/#id2):

 > If you are writing your own AJAX detection method, request.is_ajax() can be reproduced exactly as request.headers.get('x-requested-with') == 'XMLHttpRequest'.

Author: gtwohig

beeline/middleware/django/__init__.py

@@ -97,7 +97,7 @@ def get_context_from_request(self, request):
             "request.scheme": request.scheme,
             "request.secure": request.is_secure(),
             "request.query": request.GET.dict(),
-            "request.xhr": request.is_ajax(),
+            "request.xhr": request.headers.get('x-requested-with') == 'XMLHttpRequest',
         }
 
     def get_context_from_response(self, request, response):
@@ -183,6 +183,6 @@ def get_context_from_request(self, request):
             "request.scheme": request.scheme,
             "request.secure": request.is_secure(),
             "request.query": request.GET.dict(),
-            "request.xhr": request.is_ajax(),
+            "request.xhr": request.headers.get('x-requested-with') == 'XMLHttpRequest',
             "request.post": request.POST.dict(),
         }