Bean properties with Optional fields cannot be saved

[ljacqu]

As detected while working on AuthMe/AuthMeReloaded#1035, yml files can be loaded just fine for a bean class with Optional properties, but writing it back is not possible:

ch.jalu.configme.beanmapper.ConfigMeMapperException: Class 'class java.util.Optional' has no writable properties

	at ch.jalu.configme.beanmapper.leafproperties.LeafPropertiesGenerator$EntryBuilder.collectPropertiesFromBean(LeafPropertiesGenerator.java:55)
	at ch.jalu.configme.beanmapper.leafproperties.LeafPropertiesGenerator$EntryBuilder.collectPropertyEntries(LeafPropertiesGenerator.java:88)
	at ch.jalu.configme.beanmapper.leafproperties.LeafPropertiesGenerator$EntryBuilder.collectPropertiesFromBean(LeafPropertiesGenerator.java:59)
	at ch.jalu.configme.beanmapper.leafproperties.LeafPropertiesGenerator$EntryBuilder.collectPropertyEntries(LeafPropertiesGenerator.java:88)
	at ch.jalu.configme.beanmapper.leafproperties.LeafPropertiesGenerator$EntryBuilder.collectPropertyEntries(LeafPropertiesGenerator.java:81)
	at ch.jalu.configme.beanmapper.leafproperties.LeafPropertiesGenerator$EntryBuilder.collectPropertiesFromBean(LeafPropertiesGenerator.java:59)
	at ch.jalu.configme.beanmapper.leafproperties.LeafPropertiesGenerator.generate(LeafPropertiesGenerator.java:32)
	at ch.jalu.configme.resource.YamlFileResource.convertPropertiesToExportableTypes(YamlFileResource.java:169)
	at ch.jalu.configme.resource.YamlFileResource.exportProperties(YamlFileResource.java:116)
	at ch.jalu.configme.SettingsManager.save(SettingsManager.java:143)
	at ch.jalu.configme.SettingsManager.validateAndLoadOptions(SettingsManager.java:153)
	at ch.jalu.configme.SettingsManager.<init>(SettingsManager.java:68)
	at ch.jalu.configme.SettingsManager.<init>(SettingsManager.java:53)