Add QHttpHeaders API to replace() a header

While porting Qt internals to use QHttpHeaders class, this has been a recurring pattern:

h.removeAll(QHttpHeaders::WellKnownHeader::ContentType);
h.append(QHttpHeaders::WellKnownHeader::ContentType, "application/x-empty");

In other words; remove all possible header 'name' values, and then add a new value, and then append a new 'name' + 'value'.

Implementing this is a small task, but first some thought needs to go into what's the API like.