Add support for Warning headers?

[zolyfarkas]

It would be useful to support:

https://tools.ietf.org/html/rfc7234#section-5.5

[andrey-qlogic]

The code already supports any user defined header public HttpHeaders set(String fieldName, Object value), if it not enough we could define "Warning" header as a class field.

@key("Warning")
private List warning;