setTypeFields bug

[caizx666]

fix Error: Schema must contain unique named types but contains multiple types named

function setTypeFields(type, fields) {
type._typeConfig.fields = function () {
return fields;
};
}

change to

function setTypeFields(type, fields) {
type._fields = null; // for reset
type._typeConfig.fields = function () {
return fields;
};
}