fix: if-match and if-none-match header are present on the same request

[kesslerd]

Calling setEtag adds the header without removing previously added headers. This can lead to having multiples of the same header or conflicting headers like if-match='*' and if-none-match='*'

The error happens during the call writeCalendarOnServer when the parameter isNew is false

1. The AbstractPutMethod constructor calls setEtag(null)
2. The put method in the DefaultDavClientcalls setEtag("*")