Fix merging fields with arguments

executeField shouldn't assume that a selection has only one field with a
given name, but it should take the first field. The underlying cause is
a wrong pattern, which (because of the laziness) is executed only if the
field has arguments.

src/Language/GraphQL/Execute/Execution.hs

@@ -97,7 +97,7 @@ executeField fieldResolver prev fields
   where
     executeField' fieldDefinition resolver = do
         let Out.Field _ fieldType argumentDefinitions = fieldDefinition
-        let (Transform.Field _ _ arguments' _ _ :| []) = fields
+        let Transform.Field _ _ arguments' _ _ = NonEmpty.head fields
         case coerceArgumentValues argumentDefinitions arguments' of
             Nothing -> addError null $ Error "Argument coercing failed." [] []
             Just argumentValues -> do