[3.0] Remove default content-length header from MultipartStream class

[Procionegobbo]

Content-Length in multipart bodies is not necessary. As far as I can verify is not used in any browser or other HTTP client.

For reference

• Similar problems in java package okhttp: https://github.com/square/okhttp/pull/7319
• stackoverflow question: https://stackoverflow.com/questions/54299044/remove-content-length-header-in-guzzle-multipart-request