add tests to preserve existing Set nil behavior

While it may not be intuitive, providers expect that setting a `nil`
value will appear as an empty string in state.

builtin/providers/test/resource.go

@@ -202,6 +202,14 @@ func testResourceRead(d *schema.ResourceData, meta interface{}) error {
 		d.Set("set", []interface{}{})
 	}
 
+	// This mimics many providers always setting a *string value.
+	// The existing behavior is that this will appear in the state as an empty
+	// string, which we have to maintain.
+	o := d.Get("optional")
+	if o == "" {
+		d.Set("optional", nil)
+	}
+
 	// This should not show as set unless it's set in the config
 	_, ok := d.GetOkExists("get_ok_exists_false")
 	if ok {

builtin/providers/test/resource_test.go

@@ -1050,3 +1050,39 @@ resource "test_resource" "foo" {
 		},
 	})
 }
+
+func TestResource_unsetNil(t *testing.T) {
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+	required = "yep"
+	required_map = {
+	    key = "value"
+	}
+	optional = "a"
+}
+				`),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("test_resource.foo", "optional", "a"),
+				),
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+	required = "yep"
+	required_map = {
+	    key = "value"
+	}
+}
+				`),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("test_resource.foo", "optional", ""),
+				),
+			},
+		},
+	})
+}