Use canonical case for ETag response header (COUCHDB-3134)
diff --git a/src/couch_mrview_show.erl b/src/couch_mrview_show.erl
index f7e0f56..0e5217c 100644
--- a/src/couch_mrview_show.erl
+++ b/src/couch_mrview_show.erl
@@ -323,7 +323,7 @@
     % Vary headers. If the external function controls the Etag, we'd have to
     % run it to check for a match, which sort of defeats the purpose.
     apply_headers(ExternalResponse, [
-        {<<"Etag">>, CurrentEtag},
+        {<<"ETag">>, CurrentEtag},
         {<<"Vary">>, <<"Accept">>}
     ]).