public async Task reject_invalid_grant_type()
{
var server = new TestServerBuilder()
.WithSuccessAuthentication()
.Build();
var response = await server.CreateClient().PostAsync("/token", GrantTypes.AnInvalidGrantType());
var content = await response.Content.ReadAsStringAsync();
content.Should().Be(ServerMessages.InvalidGrantType);
response.StatusCode.Should().Be(StatusCodes.Status400BadRequest);
}