HTTP - No need extra newline before the endBoundaryStream in writeMultipart

[kphuanghk]

Regarding #1397 , after introduced newline in MultipartItem stream construction. It should no longer need to have newline before the endBoundaryStream . This will cause extra 2 bytes "/r/n" to the last multipart item.

Original:

        let wholePayload = Seq.append segments [newlineStream(); endBoundaryStream; ]

Shall be:

        /// per spec, close-delimiter := "--" boundary "--" CRLF ; no need extra newline
        let wholePayload = Seq.append segments [ endBoundaryStream; ]

Here's the skeleton program I used to test the two extra bytes issue:
TestProgram.fs.MD

For those affected by this issue, just need to remove extra two bytes for the affected files.

[cartermp]

Addressed in #1421