headers property Null safety

1. smh_flutter_sdk
2. smh_base_entity
3. SMHResponse<T>
4. headers property

SMHResponse class

1. Constructors
2. SMHResponse
3. fromJson
4. Properties
5. data
6. hashCode
7. headers
8. runtimeType
9. statusCode
10. statusMessage
11. Methods
12. noSuchMethod
13. toJson
14. toString
15. Operators
16. operator ==