Headers should be iterable

[ChALkeR]

% boa
>> [...new Headers([['x','y']])]
Uncaught: TypeError: value with type `object` is not iterable (native at core/engine/src/builtins/iterable/mod.rs:307:13)
    at <main> (unknown at :?:?)
>> new Map(new Headers([['x','y']]))
Uncaught: TypeError: value with type `object` is not iterable (native at core/engine/src/builtins/iterable/mod.rs:307:13)
    at <main> (unknown at :1:1)

% node
> [...new Headers([['x','y']])]
[ [ 'x', 'y' ] ]
> new Map(new Headers([['x','y']]))
Map(1) { 'x' => 'y' }