Replaced _make_url_absolute with URI::new_abs()

Patch provided by Josef Toman via rt.cpan.org
(https://rt.cpan.org/Public/Bug/Display.html?id=84436)
Fixed header handling to use header_field_names()

Patch provided by Josef Toman via rt.cpan.org
(https://rt.cpan.org/Public/Bug/Display.html?id=84429)
Updated THANKS to thank Josef Toman
Bump to v0.15 (plus CHANGES update)