SignOut all identity cookies except for RememberMe

Fixes https://github.com/aspnet/Identity/issues/211

src/Microsoft.AspNet.Identity/SignInManager.cs

@@ -87,6 +87,8 @@ namespace Microsoft.AspNet.Identity
         public virtual void SignOut()
         {
             Context.Response.SignOut(IdentityOptions.ApplicationCookieAuthenticationType);
+            Context.Response.SignOut(IdentityOptions.ExternalCookieAuthenticationType);
+            Context.Response.SignOut(IdentityOptions.TwoFactorUserIdCookieAuthenticationType);
         }
 
         private async Task<bool> IsLockedOut(TUser user, CancellationToken token)

test/Microsoft.AspNet.Identity.Test/SignInManagerTest.cs

@@ -485,6 +485,8 @@ namespace Microsoft.AspNet.Identity.Test
             var response = new Mock<HttpResponse>();
             context.Setup(c => c.Response).Returns(response.Object).Verifiable();
             response.Setup(r => r.SignOut(authenticationType)).Verifiable();
+            response.Setup(r => r.SignOut(IdentityOptions.TwoFactorUserIdCookieAuthenticationType)).Verifiable();
+            response.Setup(r => r.SignOut(IdentityOptions.ExternalCookieAuthenticationType)).Verifiable();
             var contextAccessor = new Mock<IContextAccessor<HttpContext>>();
             contextAccessor.Setup(a => a.Value).Returns(context.Object);
             var roleManager = MockHelpers.MockRoleManager<TestRole>();