cv-qualifier should be removed before checking the Header concept

C++11 allows this kind of construction:

    connection->set_status(Connection::bad_request);
    const auto size_to_send = std::to_string(reason.size());
    auto headers = { Header{"Content-Length", size_to_send} };
    auto range = boost::make_iterator_range(headers);
    connection->set_headers(range);
    connection->write(reason);

But in the case of an initializer_list, elements are const, then the actual
type tested is "const ValueType" instead of "ValueType".
Merge pull request #347 from daedric/remove_cv_qualifier

cv-qualifier should be removed before checking the Header concept