[1.8.x] Split DTL context creation into its own function.

This reduces the length of rope RequestContext gives users to hang
themselves with.

Thanks Alex Hill for the report and Tim Graham for the review.

Backport of f2c104a from master.

django/template/backends/django.py

@@ -4,7 +4,7 @@ from __future__ import absolute_import
 import warnings
 
 from django.conf import settings
-from django.template.context import Context, RequestContext
+from django.template.context import Context, RequestContext, make_context
 from django.template.engine import Engine, _dirs_undefined
 from django.utils.deprecation import RemovedInDjango20Warning
 
@@ -69,14 +69,6 @@ class Template(object):
                 RemovedInDjango20Warning, stacklevel=2)
 
         else:
-            if request is None:
-                context = Context(context)
-            else:
-                # The following pattern is required to ensure values from
-                # context override those from template context processors.
-                original_context = context
-                context = RequestContext(request)
-                if original_context:
-                    context.push(original_context)
+            context = make_context(context, request)
 
         return self.template.render(context)

django/template/context.py

@@ -234,3 +234,19 @@ class RequestContext(Context):
         if hasattr(new_context, '_processors_index'):
             del new_context._processors_index
         return new_context
+
+
+def make_context(context, request=None):
+    """
+    Create a suitable Context from a plain dict and optionally an HttpRequest.
+    """
+    if request is None:
+        context = Context(context)
+    else:
+        # The following pattern is required to ensure values from
+        # context override those from template context processors.
+        original_context = context
+        context = RequestContext(request)
+        if original_context:
+            context.push(original_context)
+    return context