multipart/form-data must use CRLF line breaks

[kalekseev]

Fixes #1313
Environment.NewLine equals \n on unix like systems that leads to invalid payload with LF line breaks:

--define a custom boundary here<LF>
Content-Disposition: form-data; name="images"; filename="image0.png"<LF>
Content-Type: image/png<CRLF>
...

From https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html

The boundary must be followed immediately either by another CRLF and the header fields for the next part, or by two CRLFs, in which case there are no header fields for the next part (and it is therefore assumed to be of Content-Type text/plain).

[cartermp]

I think this looks sensible. @baronfel, since you added the feature, any concerns?

[baronfel]

None at all, excellent spot @kalekseev

[cartermp]

Thanks @kalekseev