Add httpContentType to inputStreamReadable (#35)

* Add httpContentType to inputStreamReadable

Since InputStreams manage binary data, we can default it to `application/octet-stream` the same as `Array[Byte]`s or `ByteBuffer`s

* Update geny/src/geny/Writable.scala

Co-authored-by: Lorenzo Gabriele <lorenzolespaul@gmail.com>

geny/src/geny/Writable.scala

@@ -134,5 +134,6 @@ object Readable{
 
   implicit class InputStreamReadable(i: InputStream) extends Readable{
     def readBytesThrough[T](f: InputStream => T): T = f(i)
+    override def httpContentType: Option[String] = Some("application/octet-stream")
   }
 }