get_header_value performs double lookup on unordered_multimap

[mispritt]

In http_request.h, get_header_value is currently implemented as:

if (headers.count(key))
{
    return headers.find(key)->second;
}

When headers is a ci_map (std::unordered_multimap), this results in two independent
lookups. In particular, count() may traverse all elements with the given
key, followed by another lookup in find().

Unless there is a specific reason for this, the same behaviour can be
achieved with a single lookup:

auto it = headers.find(key);
if (it != headers.end())
{
    return it->second;
}