fix: manage Headers instances in the JS kit

[Angelmmiguel]

Properly manage Headers instances by reusing them when a Response receives it. Before, a Headers object was causing a { headers: { headers: {} } object, which was wrong. I also took the opportunity to improve the Headers parsing by only allowing valid types (Object or Headers), and valid header values (string).

It fixes #233

[ereslibre]

Thanks for this fix :)