Allow unlinking of billing account. (hashicorp#138)

* Allow unlinking of billing account. Closes hashicorp#133

* Add acceptance test for unlinking the billing account.

* Just apply the resource definition without the billing account instead of setting an empty billing account.

google/resource_google_project.go

@@ -200,8 +200,9 @@ func resourceGoogleProjectUpdate(d *schema.ResourceData, meta interface{}) error
 	// Billing account has changed
 	if ok := d.HasChange("billing_account"); ok {
 		name := d.Get("billing_account").(string)
-		ba := cloudbilling.ProjectBillingInfo{
-			BillingAccountName: "billingAccounts/" + name,
+		ba := cloudbilling.ProjectBillingInfo{}
+		if name != "" {
+			ba.BillingAccountName = "billingAccounts/" + name
 		}
 		_, err = config.clientBilling.Projects.UpdateBillingInfo(prefixedProject(pid), &ba).Do()
 		if err != nil {

google/resource_google_project_test.go

@@ -115,6 +115,13 @@ func TestAccGoogleProject_updateBilling(t *testing.T) {
 					testAccCheckGoogleProjectHasBillingAccount("google_project.acceptance", pid, billingId2),
 				),
 			},
+			// Unlink the billing account
+			resource.TestStep{
+				Config: testAccGoogleProject_create(pid, pname, org),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckGoogleProjectHasBillingAccount("google_project.acceptance", pid, ""),
+				),
+			},
 		},
 	})
 }