when an event is modified, its creation timestamp is not preserved

[hannesm]

The Webdav_api.write_component is called by the PUT handler on calendar resources. This in turns calls write_if_parent_exists which creates a fresh property map (Properties.create) with the current timestamp as creation time. It would be nicer if the property map of the original event (if existing) would be preserved -- the modification timestamp is updated in any case).

As example, please take a look at event change, and its accompanying prop change (NB the latter also removed getetag and getlastmodified due to c4f0aa0 and 95cdf78).