Allow to set an forced xml namespace per item

In some conditions it is necessary to change the XML namespace within the SOAP request. For example from NS_EWS_MESSAGES to NS_EWS_TYPES.

Unfortunately Nokogiri does not allow that if the namespace is already known in the XML document. So I needed to enforce a namespace in order to pass Exchange's schema validation.

See also: https://github.com/sparklemotion/nokogiri/issues/925