Javadoc
Adds an header element to be contributed to the page.
If keyHint argument is provided and if a header element
with the provided key hint already exists, then
the element will be overwritten to the item.
If the element is null and keyHint is not null,
the element with the keyHint is removed from the response.
If the element is not null and keyHint is null,
then the keyHint will be automatically generated by the element content.
Meanwhile, the element should implement java.io.Serializable.
Otherwise it will throw a java.io.NotSerializableException.