insert -fgl +http

insertHeader :: HasHeaders a => HeaderSetter a
HTTP Network.HTTP.Headers
insertHeader hdr val x inserts a header with the given header name and value. Does not check for existing headers with same name, allowing duplicates to be introduce (use replaceHeader if you want to avoid this.)
insertHeaderIfMissing :: HasHeaders a => HeaderSetter a
HTTP Network.HTTP.Headers
insertHeaderIfMissing hdr val x adds the new header only if no previous header with name hdr exists in x.
insertHeaders :: HasHeaders a => [Header] -> a -> a
HTTP Network.HTTP.Headers
insertHeaders hdrs x appends multiple headers to x's existing set.