Merge pull request #580 in ZP/z-push from feature/ZP-1271-impersonate-shared-folders-with-own to develop

* commit '593e8ae3':
  ZP-1271 Fixed whitespace.
  ZP-1271 Fixed typo & whitespace.
  ZP-1271 Overwrite user only if no specific username was requested.
  ZP-1271 Impersonating support for the Kopano backend. Send-As with OL working correctly.
  ZP-1271 Correctly set exporter to false if root can not be determined for hierarchy export.