Fix ICal data encoding to match the Content-Type header

Some CalDAV servers (SOGo at least) seem a bit picky
about the data encoding and if the ICal data encoding
does not match the one specified in the Content-Type
HTTP header then the creation or update is rejected.
Bug is exhibited when any ICal property contains
non-ASCII characters.

Testing was performed with the latest SOGo stable
release.
Merge pull request #33 from goestreicher/master

Fix ICal data encoding to match the Content-Type header