draft-ietf-httpbis-header-structure-15, 3.2. Dictionaries

3.2.  Dictionaries
https://tools.ietf.org/html/draft-ietf-httpbis-header-structure-15

|   A dictionary with a mix of singular and list values, some with
|   parameters:
|
|   Example-MixDict: a=(1 2), b=3, c=4;aa=bb, d=(5 6);valid=?1


but

3.1.2.  Parameters
https://tools.ietf.org/html/draft-ietf-httpbis-header-structure-15#section-3.1.2

|   Parameters whose value is Boolean true MUST omit that value when
|   serialised.  For example:
|
|   Example-IntHeader: 1; a; b=?0

/ Kari Hurtta