[Bug Report] Nested el-form, required attributes is Inherited

[JinboMa]

Element UI version

2.4.6

OS/Browsers version

maxOS 10.13.6 / chrome 68.0.3440.106

Vue version

2.5.17

Reproduction Link

https://jsfiddle.net/zh1e0kc9/6/

Steps to reproduce

1. add a el-form component A
2. put a el-form-item component under A
3. the el-form-item component add required attribute
4. add another el-form B
5. put B under the el-form-item component
6. put el-form-item component under B
7. the el-form-item isn't add required attribute

What is Expected?

the el-form-item have not * at B

What is actually happening?

the el-form-item have * at B