#574 provide helper function to get http headers

[Jeff Rogers]

The http package puts the response headers into the meta element of the state array exactly as they are received from the server. If a program needs to get a specific well-known header (i.e., Location) they must search the meta array in a case-insensitive manner rather than just accessing the keys.

The requested feature is to provide a case-insensitive lookup into the meta array, either through a new helper function (::http::header) or by having http::meta take an optional key argument to return only the requested key instead of the entire array. (::http::meta token ?key?)

[Alexandre Ferrieux]

A new helper function ::http::header would indeed be the simplest, returning basically the same key-value list (isomorphous to a dict BTW) as ::meta but with [string tolower] applied to the keys.