Allow map binding to work with unescaped characters

Spring boot version： 2.0.2 (1.5.13 is normal)

Problem Description：

If the map key is with “/”, it cannot be parsed normally. Others are normal.

Example：

application.yml 

shiro:
  testMap:
    /test1: test1
    /test2: test2

Configuration class

@Data
@ConfigurationProperties(prefix="shiro")
public class ShiroProperties {
	private Map<String,String> testMap;
}

Test class

@Transactional
@RunWith(SpringRunner.class)
@SpringBootTest(classes=DataManage.class)
public class ShiroTest extends BaseTransactionalTest{

	@Autowired
	private ShiroProperties shiroConfig;
	
	@Test
	public void read() {
		System.out.println(shiroConfig.toString());
	}
	
}

console info：

ShiroProperties(testMap={test1=test1, test2=test2})

Start with the application.yml example and the ShiroProperties and ShiroTest reproducer shown in the issue. Run the binding test against the affected Spring Boot version, then trace the configuration-properties map binding behavior. Done means map keys containing unescaped '/' are retained while existing keys continue to bind normally.