provide multibyte aware helper functions mb_str_replace, mb_substr_replace and mb_array_change_key_case for handling with UTF 8

webdav client crashes for secure connections if he doesn't have the right root certificate for ssl verification. For the moment I print at least a useful error message to the OC log and the Apache log

check for resource

Revert "bugfix for #1098 (assigning a group to a user, the group appears incrementally many times the more you assign it)"

This reverts commit 265d7e50.