Add file encoding

[yangm97]

I see this lib is capable of decoding files but not encode them.

[Tieske]

see usage:

-- Get a multipart/form-data representation of the object
local body = multipart_data:tostring()

[yangm97]

I guess I was not clear. Suppose I have a file path or a userdata object, there's no way to encode that into a POST request. :set_simple will only work for strings.

[kikito]

If you have a file path, you can read it into a string and encode it using plain Lua:

local f = assert(io.open(path, "rb"))
local str = f:read("*a")
f:close()

In the case of userdata object, they really need to expose some sort of :tostring facility as each userdata can come from a different place/library. There is no way to program a "generic userdata encoder/decoder".