Fix for InsertAsync in PostgresAdapter (#689)

* Fix for PostgresAdapter in SqlMapperExtensions.Async. Copied same line of code from SqlMapperExtensions to fix RuntimeBinderException on InsertAsync
* Cleanup

Dapper.Contrib/SqlMapperExtensions.Async.cs

@@ -458,10 +458,9 @@ public async Task<int> InsertAsync(IDbConnection connection, IDbTransaction tran
 
         // Return the key by assinging the corresponding property in the object - by product is that it supports compound primary keys
         var id = 0;
-        var values = results.First();
         foreach (var p in propertyInfos)
         {
-            var value = values[p.Name.ToLower()];
+            var value = ((IDictionary<string, object>)results.First())[p.Name.ToLower()];
             p.SetValue(entityToInsert, value, null);
             if (id == 0)
                 id = Convert.ToInt32(value);