Re: postContent with custom header

[dom96]

There is no need for this procedure to be exported. You should use the `post` 
procedure which uses this `format` proc: 
[https://github.com/nim-lang/Nim/blob/master/lib/pure/httpclient.nim#L1197](https://github.com/nim-lang/Nim/blob/master/lib/pure/httpclient.nim#L1197)