Empty string was printed after tostring() call

[example.txt](https://github.com/Kong/lua-multipart/files/9418657/example.txt)
Hi!
My file example.txt is attached.
```lua
local Multipart = require("multipart")
local io = require("io")
local open = io.open
local function read_file(path)
local file = open(path, "r")
if not file then return nil end
local content = file:read "*a"
file:close()
return content
end
local body = read_file("example.txt");
local multipart_data = Multipart(body, "text/html")
local t = multipart_data:tostring()
print (t)
```
This code prints empty line. What's wrong? Please, explain it.

Reproduce the report using the attached example.txt and the Lua snippet, starting at the Multipart(body, "text/html") call and the tostring() entry point. Trace how the input is parsed and determine why print(t) produces an empty line; done means the cause and expected usage are clearly explained.

Written by the indexing model from the issue text.