Fix simpleType length restriction

[tishkinaa]

If simpleType has only length restriction then it will skipped when generating structs, because flatten1 method returns builtin type if simpleType has got empty doc field.

https://github.com/droyo/go-xml/blob/master/xsdgen/xsdgen.go#L466

https://github.com/droyo/go-xml/blob/master/xsdgen/xsdgen.go#L534

it's represents in broken ExamplePackageName https://github.com/droyo/go-xml/blob/master/xsdgen/example_test.go#L117