LPS-28985 Manage Header with HashSet

[shuyangzhou]

No description provided.

[rotty3000]

@shuyangzhou regarding your question about

List<Header> headerValues = ListUtil.copy(entry.getValue());

It was because headers were being cleared by subsequent code, since the List is a reference, and clearing values from the list cleared them from the response.

If you tested to make sure the headers (of all types) set in a portlet (including plugins) actually arrive at the client, I'm ok with your change.

[shuyangzhou]

@rotty3000

The List from entry.getValue() is only used within this for loop. After the loop all headers are putted into response object, no matter you clear the list or not, it won't affect the response object.

[brianchandotcom]

Merged. Thank you.
View total diff: 5fe237f...7cbd0cc