Do not add WHERE clause if only param is order

Fixes #119

postgrest.cabal

@@ -1,5 +1,5 @@
 name:                  postgrest
-version:               0.2.4.9
+version:               0.2.4.10
 synopsis:              The database is your api
 license:               MIT
 license-file:          LICENSE

src/PgQuery.hs

@@ -43,7 +43,7 @@ limitT r q =
 
 whereT :: Net.Query -> StatementT
 whereT params q =
- if L.null params
+ if L.null cols
    then q
    else q <> (" where ",[],mempty) <> conjunction
  where

test/Feature/QuerySpec.hs

@@ -39,6 +39,9 @@ spec = beforeAll (clearTable "items" >> createItems 15)
         , matchHeaders = ["Content-Range" <:> "0-1/2"]
         }
 
+    it "without other constraints" $
+      get "/items?order=asc.id" `shouldRespondWith` 200
+
   describe "Canonical location" $ do
     it "Sets Content-Location with alphabetized params" $
       get "/no_pk?b=eq.1&a=eq.1"