A target selector can be a label name, version name, working resource id,
or an indicator that the VersionedResource itself should be selected.
A TargetSelector selects the indicated revision.
Get the selector key for this TargetSelector. In this case,
the versioned resource itself is selected rather than a specific
revision.
the Target-Selector key exception: com.ibm.webdav.WebDAVException -
Fields inherited from com.ibm.webdav.TargetSelector