nitg: Changed specification of 'for .. do' statements

Can now iterate over every type that provides an iterator method.

Method iterator must return an instance of Iterator[E] of MapIterator[K, E]

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib: Implemented HeaderMapIterator on Curl

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>