Avoid panic if printer is off or unreachable for some reason.

Closes #16
Closes #17
Closes #15

packages/ipp/response.go

@@ -52,6 +52,10 @@ func (r Response) String() string {
 	return r.header.String() + "\n" + r.a.String()
 }
 
+func (r Response) Header() ippResponseHeader {
+	return r.header
+}
+
 // Marshal converts the response object to a wire formatted byte stream.
 func (r *Response) Marshal() []byte {
 	a := make([]byte, 0, 20)