forEach abstract method Null safety
Performs the action
on each header.
The action
function is called with each header's name and a list
of the header's values. The casing of the name string is determined by
the last add or set operation for that particular header,
which defaults to lower-casing the header name unless explicitly
set to preserve the case.
Implementation
void forEach(void Function(String name, List<String> values) action);