Defaults for minOccurs/maxOccurs on ListElement should match the XML spec

[FelixSchwarz]

Currently the default values for minOccurs/maxOccurs on the ListElement are none. I think this means the attributes are omitted when rendering a WSDL/XML schema but at least for maxOccurs this means you can add too many items to the element resulting in an invalid state.

We should make sure that our defaults are the same as the XML spec (bonus: try to keep the generated WSDL/XML schema minimal, omitting the attribute if it equals 1). For minOccurs we probably can't enforce the restriction right away because we need a way to start with an empty list and append values but maxOccurs should be always checked.

Maybe we should also warn/raise if we are about to render XML but the amount of items does not match the occurrence constraints?