Adds a header value. The header named name
will have the value value
added to its list of values. Some headers are single valued, and for these adding a value will replace the previous value. If the value is of type DateTime a HTTP date format will be applied. If the value is a List
each element of the list will be added separately. For all other types the default toString
method will be used.
void add(String name, Object value);
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Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dart.dev/stable/2.5.0/dart-io/HttpHeaders/add.html