Update existing vary headers in response instead of overwriting them.

Previously, any existing vary headers would simply be wiped out by DRF. Using patch_vary_headers assures that existing headers remain.

rest_framework/views.py

@@ -9,6 +9,7 @@ from django.db import models
 from django.http import Http404
 from django.http.response import HttpResponseBase
 from django.utils import six
+from django.utils.cache import cc_delim_re, patch_vary_headers
 from django.utils.encoding import smart_text
 from django.utils.translation import ugettext_lazy as _
 from django.views.decorators.csrf import csrf_exempt
@@ -414,6 +415,11 @@ class APIView(View):
             response.accepted_media_type = request.accepted_media_type
             response.renderer_context = self.get_renderer_context()
 
+        # Add new vary headers to the response instead of overwriting.
+        vary_headers = self.headers.pop('Vary', None)
+        if vary_headers is not None:
+            patch_vary_headers(response, cc_delim_re.split(vary_headers))
+
         for key, value in self.headers.items():
             response[key] = value