Map of HTTP header key and value(s) associated with them. Since HTTP headers can contain duplicate keys, this structure can return a list of values associated with a single key.
Construct a fresh map of HTTP headers with a single key and value entry
Retrieve a key from a header. If the header is one of the set of headers defined to have list values, then all of the values are concatenated into a single string separated by commas and returned. If it is a singleton header, then the first value is selected and no concatenation is performed.