Add UpdateFolder operation
Add UpdateFolder operation

The update class now has two different to_xml() functions.
Since Exchange distinguishes between folder and item operations
and has different XML tags there.

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