Suggested implementation of MultiPart for http4s

gistfile1.scala

final case class Name(value:String) extends AnyVal
sealed trait Part
final case class FormData(name:Name,
                          content: Entity,
                          contentType: Option[ContentType] = None ) extends Part
                          
final case class MultiPart(parts: Seq[Part]) {
  
  def boundary = ??? // leaving out implementation for clarity 
  
  def header   = ??? // leaving out implementation for clarity 
}
/*
   E.g of creating a simple multipart 
*/
   val txtToEntity: String => Entity = ???
    
   val entity = txtToEntity("I Am a textual entity.")
   val field1 = FormData("field1", entity)

   val multiPart = MultiPart(List(field1))
  
/*
  Usuage will depend  on whether changes to EntityEncoder are possible.
   
  Without changes to EntityEncoder : 
*/
    request.withHeaders(multiPart.headers).withBody(multiPart)
/*
  With changes to EntityEncoder : 
*/
request.withBody(multiPart)

/*
 * The reason to change EntityEncoder, is that the request headers and entity body are dependent 
 * on the Multipart boundary, which is currently only available to the 'toEntity' method. 
 * 
 */

Allowing the toEntity method to generate headers seams reasonable, in theory. I think that would be a parallel or followup issue/PR.

Makes sense. I'll try and get a PR to you for the current work.