Cannot round-trip to string with a dyn uti

[psexton]

For any Uti object returned by UtiBuilder, calling Uti#toString followed by UtiBuilder#fromString should return a Uti object equal to the original. This is not the case for a dyn.* Uti, as evidenced by this failing test:

String unknownSuffix = "foobar";
UtiBuilder instance = new UtiBuilder();
Uti orig = instance.fromSuffix(unknownSuffix);
Uti copy = instance.fromString(orig.toString());

copy will be null.