resource/aws_db_instance: Prevent InvalidParameterCombination: No mod…

…ifications were requested error when updating only allow_major_version_upgrade argument

Reference: #500

Previous output from updated acceptance testing:

```
--- FAIL: TestAccAWSDBInstance_AllowMajorVersionUpgrade (491.43s)
    testing.go:568: Step 2 error: errors during apply:

        Error: Error modifying DB Instance tf-acc-test-4576888905000364877: InvalidParameterCombination: No modifications were requested
```

Output from acceptance testing:

```
--- PASS: TestAccAWSDBInstance_AllowMajorVersionUpgrade (433.02s)
```

aws/resource_aws_db_instance.go

@@ -1470,7 +1470,8 @@ func resourceAwsDbInstanceUpdate(d *schema.ResourceData, meta interface{}) error
 	if d.HasChange("allow_major_version_upgrade") {
 		d.SetPartial("allow_major_version_upgrade")
 		req.AllowMajorVersionUpgrade = aws.Bool(d.Get("allow_major_version_upgrade").(bool))
-		requestUpdate = true
+		// Having allowing_major_version_upgrade by itself should not trigger ModifyDBInstance
+		// InvalidParameterCombination: No modifications were requested
 	}
 	if d.HasChange("backup_retention_period") {
 		d.SetPartial("backup_retention_period")

aws/resource_aws_db_instance_test.go

@@ -293,6 +293,13 @@ func TestAccAWSDBInstance_AllowMajorVersionUpgrade(t *testing.T) {
 					"skip_final_snapshot",
 				},
 			},
+			{
+				Config: testAccAWSDBInstanceConfigAllowMajorVersionUpgrade(rName, false),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSDBInstanceExists(resourceName, &dbInstance1),
+					resource.TestCheckResourceAttr(resourceName, "allow_major_version_upgrade", "false"),
+				),
+			},
 		},
 	})
 }