POSTing a String via <<(String) defaults to UTF-8 encoded "text/plain"

- adds setBodyEncoding(charset: String)
- adds setContentType(mediaType: String, charset: String)
- <<(String) defaults to text/plain; charset=UTF-8
setContentType sets body encoding too
Merge branch 'topic/uft8body' of github.com:eed3si9n/reboot

Conflicts:
	core/src/main/scala/requests.scala