When initializing a YamlFactory as follows, negative numbers are not quoted correctly.
YAMLFactory factory = new YAMLFactory();
factory.enable(YAMLGenerator.Feature.MINIMIZE_QUOTES);
factory.enable(YAMLGenerator.Feature.ALWAYS_QUOTE_NUMBERS_AS_STRINGS);
Serializing 60 currently results in "60".
Serializing -14 currently results in 14.