http_response: parse XML payload lazily upon first access
http_response: parse XML payload lazily upon first access

Also add scope guard helper class. (Would use BOOST_SCOPE_EXIT_ALL if we
would use boost, though.)

m-Schlitzer/ews-cpp,otris/ews-cpp,otris/ews-cpp,m-Schlitzer/ews-cpp,m-Schlitzer/ews-cpp,bkircher/ews-cpp,bkircher/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,otris/ews-cpp