Do not supply a default content type.

Hello.

I would like to know if you would welcome a change to remove suppy_default_content_type.

Checking RFC for Content-Type: https://datatracker.ietf.org/doc/html/rfc7231#section-3.1.1.5

   A sender that generates a message containing a payload body SHOULD
   generate a Content-Type header field in that message unless the
   intended media type of the enclosed representation is unknown to the
   sender.  If a Content-Type header field is not present, the recipient
   MAY either assume a media type of "application/octet-stream"
   ([RFC2046], Section 4.5.1) or examine the data to determine its type.

Based on the RFC, I think Net::HTTP behavior is incorrect - assuming a default content type is not correct because the media type is not known unless Net::HTTP reads/inspects the body. The receiver may assume octet-stream unless provided.

This has caused issues with AWS services, where content type may be a modeled API parameter in REST services, like with S3 where you can specify the content type of an object. Currently we work around this with a patch:

Thread.current[:net_http_skip_default_content_type] = true

def self.apply!
  Net::HTTPGenericRequest.prepend(PatchDefaultContentType)
end

module PatchDefaultContentType
  def supply_default_content_type
    return if Thread.current[:net_http_skip_default_content_type]

    super
  end
end

Start with supply_default_content_type in lib/net/http/generic_request.rb around line 263, then compare its behavior with the RFC 7231 guidance cited in the issue. The change is complete when Net::HTTP no longer supplies an assumed Content-Type for an unknown body, including the AWS-style use case described.