Ruby multipart refactor

[noraj]

Linked to global issue #30

ref

To send multipart/form-data use Net::HTTPHeader#set_form:

req = Net::HTTP::Post.new(uri)
req.set_form([['upload', File.open('foo.bar')]], 'multipart/form-data')

set_form

So requiring a external gem is not longer required.

<#if util.atLeastOneMultipart(requests)>
require "net/http/post/multipart"
# Psst! It require an additional gem : gem install multipart-post
</#if>

But the rest of the code need to be updated the standard library syntax instead of the external gem DSL.