Always stringigy returned HTTP status code for logging

This avoids the potential issue in which the status code would be "nil" or
"false", which HTTP client libraries may choose to signal an error condition.

matrix/api.lua

@@ -266,7 +266,7 @@ function API:_send(method, path, query_args, body, headers, api_path)
       end
       return body
    else
-      return error("HTTP " .. code .. " - " .. body)
+      return error("HTTP " .. tostring(code) .. " - " .. body)
    end
 end