The name / value pair to be parsed
An immutable pair of the lowercase name parameter states and the value it states past the first equals
sign, empty if unstated
Mapped over parseList, this reads a field keyed by name, Map resolving a repeated name last-wins as
those fields prescribe.
new Map(parseList(headers.get("cache-control")).map(directive => parseParameter(directive)));
Parses a name / value pair.
Reads one parameter of an item and equally one directive of a field stating name / value directives,
Cache-Controlamong them, the two differing only in whether the value is optional. Names are case-insensitive and reported in lowercase; a value stated as a quoted string is unquoted and unescaped, and one left unstated is reported as empty, so presence alone answers a flag.