don't send undefined etag (COUCHDB-3134)
diff --git a/src/couch_mrview_show.erl b/src/couch_mrview_show.erl
index 0e5217c..52e07a7 100644
--- a/src/couch_mrview_show.erl
+++ b/src/couch_mrview_show.erl
@@ -317,6 +317,8 @@
     Acc.
 
 
+apply_etag(JsonResp, undefined) ->
+    JsonResp;
 apply_etag({ExternalResponse}, CurrentEtag) ->
     % Here we embark on the delicate task of replacing or creating the
     % headers on the JsonResponse object. We need to control the Etag and