Allow ByExpr and QueryExpr to remain explicit

needed to support the work done here
elixir-ecto/ecto#4417

Author: pmonson711

lib/fob/ordering.ex

@@ -24,7 +24,11 @@ defmodule Fob.Ordering do
 
   @spec config(%Query{}) :: [t()]
   def config(%Query{order_bys: orderings} = query) do
-    Enum.flat_map(orderings, fn %{expr: exprs} ->
+    Enum.flat_map(orderings, fn %expr_struct{expr: exprs}
+                                when expr_struct in [
+                                       Query.QueryExpr,
+                                       Query.ByExpr
+                                     ] ->
       config_from_ordering_expressions(exprs, query)
     end)
   end