refactor smtp sending

* [x] move package creation logic to `pmapi.SendMessageReq`
* [ ] write test of package creation logic
    * [x] internal
    * [x] plain
    * [x] external encrypted
    * [ ] signature ???
    * [x] attachments

pkg/pmapi/client.go

@@ -254,7 +254,7 @@ func (c *client) doBuffered(req *http.Request, bodyBuffer []byte, retryUnauthori
 			head += "\n"
 		}
 		c.log.Tracef("REQHEAD \n%s", head)
-		c.log.Tracef("REQBODY '%s'", string(bodyBuffer))
+		c.log.Tracef("REQBODY '%s'", printBytes(bodyBuffer))
 	}
 
 	hasBody := len(bodyBuffer) > 0