Pass request to related serializers

Related serializers may need access to the request to properly serialize
a child resource.  For example, reverse() in djangorestframework.reverse
uses request if available to return an absolute URL.  While the parent
resource has access to the request to generate the absolute URL, the
child resource does not.

djangorestframework/serializer.py

@@ -179,7 +179,8 @@ def serialize_val(self, key, obj, related_info):
             stack = self.stack[:]
             stack.append(obj)
 
-        return related_serializer(depth=depth, stack=stack).serialize(obj)
+        return related_serializer(depth=depth, stack=stack).serialize(
+            obj, request=self.request)
 
     def serialize_max_depth(self, obj):
         """
@@ -253,11 +254,15 @@ def serialize_fallback(self, obj):
         """
         return smart_unicode(obj, strings_only=True)
 
-    def serialize(self, obj):
+    def serialize(self, obj, request=None):
         """
         Convert any object into a serializable representation.
         """
 
+        # Request from related serializer.
+        if request is not None:
+            self.request = request
+
         if isinstance(obj, (dict, models.Model)):
             # Model instances & dictionaries
             return self.serialize_model(obj)