Re: postContent with custom header

[Hlaaftana]

This would work if format in httpclient was exported. For now copy and paste 
[this 
proc](https://github.com/nim-lang/Nim/blob/master/lib/pure/httpclient.nim#L438-L457)
 (and import random)
    
    
    import httpclient
    
    let data = newMultipartData({
      "data1": "value1",
      "data2": "value2",
      "data3": "value3"
    })
    
    echo format(data).body