http: Fix duplication of host/connection/user-agent request headers
http: Fix duplication of host/connection/user-agent request headers

Caused when the passed in headers have multiple keys, causing each `found` variable to depend on the iteration order of the headers table (each `found` var would only care about the last header iterated)

Example reproduction:

    local headers = { {"Front", "test"}, {"Host", "luvit.io"}, {"Behind", "test"} }
    local request = http.request({host="luvit.io", headers=headers})
    request:destroy()
    local num_hosts = 0
    for i,v in ipairs(request) do
      p(i, v)
      if v[1]:lower() == "host" then
        num_hosts = num_hosts + 1
      end
    end
    assert(num_hosts == 1)

zhaozg/luvit,kaustavha/luvit,kaustavha/luvit,bsn069/luvit,bsn069/luvit,kaustavha/luvit,luvit/luvit,luvit/luvit,bsn069/luvit,zhaozg/luvit