Streaming response body content type

[eyalfa]

streaming responses doesn't properly populate the media-type and boundary for the response's body.
This is especially an issue for MultipartMixed as it relies on the boundary in order to be able to parse the response stream into parts.

this PR introduces a test and a fix, the test initially fails with:

  Exception in thread "zio-fiber-71" java.lang.IllegalStateException: Cannot decode body as multipart/mixed without a known boundary
  	at zio.http.Body.$anonfun$asMultipartMixed$2(Body.scala:126)
  	at zio.ZIO$.$anonfun$fail$1(ZIO.scala:3164)
  	at zio.ZIO$.$anonfun$failCause$3(ZIO.scala:3173)
  	at zio.http.netty.NettyStreamBodySpec.spec(NettyStreamBodySpec.scala:153)
  	at zio.http.netty.NettyStreamBodySpec.spec(NettyStreamBodySpec.scala:104)```

applying the one-liner fix in `NettyResponse`, resolves the issue.

[eyalfa]

cc: @987Nabil