Fix conversion of http.headers in Lua 5.1 and LuaJIT

The __pairs metamethod was introduced in 5.2, so instead of using pairs()
on http.headers values, use its :each() method — which is what the __pairs
metamethod calls anyway.

matrix/httpclient/chttp.lua

@@ -23,7 +23,7 @@ end
 
 local function headers_to_dict(h)
    local headers = {}
-   for name, value in pairs(h) do
+   for name, value in h:each() do
       if name:sub(1, 1) ~= ":" then
          headers[name] = value
       end