asp-page-handler doesn't work correctly if asp-page isn't also specified

See https://github.com/DamianEdwards/RazorPagesSample/blob/master/RazorPagesWebApplication/Pages/Account/Manage/ExternalLogins.cshtml#L28

It ends up with no `action` attribute being added to the form at all.

If I use `asp-route-handler` by itself it works fine.

@pranavkm @rynowak 