provider/gce: Test updates to http_health_check

By first creating a very simple resource that mostly uses the default
values and then changing the two thresholds from their computed defaults.

This currently fails with the following error and will be fixed in a
subsequent commit:

    --- FAIL: TestAccComputeHttpHealthCheck_update (5.58s)
            testing.go:131: Step 1 error: Error applying: 1 error(s) occurred:

                    * 1 error(s) occurred:

                    * 1 error(s) occurred:

                    * Error patching HttpHealthCheck: googleapi: Error 400: Invalid value for field 'resource.port': '0'.  Must be greater than or equal to 1
                    More details:
                    Reason: invalid, Message: Invalid value for field 'resource.port': '0'.  Must be greater than or equal to 1
                    Reason: invalid, Message: Invalid value for field 'resource.checkIntervalSec': '0'.  Must be greater than or equal to 1
                    Reason: invalid, Message: Invalid value for field 'resource.timeoutSec': '0'.  Must be greater than or equal to 1

builtin/providers/google/resource_compute_http_health_check_test.go

@@ -25,6 +25,32 @@ func TestAccComputeHttpHealthCheck_basic(t *testing.T) {
 	})
 }
 
+func TestAccComputeHttpHealthCheck_update(t *testing.T) {
+	var healthCheck compute.HttpHealthCheck
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckComputeHttpHealthCheckDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccComputeHttpHealthCheck_update1,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckComputeHttpHealthCheckExists(
+						"google_compute_http_health_check.foobar", &healthCheck),
+				),
+			},
+			resource.TestStep{
+				Config: testAccComputeHttpHealthCheck_update2,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckComputeHttpHealthCheckExists(
+						"google_compute_http_health_check.foobar", &healthCheck),
+				),
+			},
+		},
+	})
+}
+
 func testAccCheckComputeHttpHealthCheckDestroy(s *terraform.State) error {
 	config := testAccProvider.Meta().(*Config)
 
@@ -83,3 +109,22 @@ resource "google_compute_http_health_check" "foobar" {
 	unhealthy_threshold = 3
 }
 `
+
+const testAccComputeHttpHealthCheck_update1 = `
+resource "google_compute_http_health_check" "foobar" {
+	name = "terraform-test"
+	description = "Resource created for Terraform acceptance testing"
+	request_path = "/not_default"
+}
+`
+
+/* Change description, restore request_path to default, and change
+* thresholds from defaults */
+const testAccComputeHttpHealthCheck_update2 = `
+resource "google_compute_http_health_check" "foobar" {
+	name = "terraform-test"
+	description = "Resource updated for Terraform acceptance testing"
+	healthy_threshold = 10
+	unhealthy_threshold = 10
+}
+`