Fix double registration of a complex type

Zend\Soap\Wsdl\ComplexTypeStrategy\DefaultComplexType::addComplexType()
stores already added classes under the methods argument `$type`.

Therefore a class with preceding backslash is treated like another complextype
than the same class without given backslash. As a result the xsd:ComplexType
definition is found twice (with exactly the same name) in the generated wsdl.