Merge pull request #532 in ZP/z-push from bugfix/ZP-1199-utf-8-truncation-results-in-invalid to develop

* commit '61cbf6a0':
  ZP-1199 For emails with a lot of HTML 3x the requested truncation on plaintext might not be enough.
  ZP-1199 Fixed variable name holding the length of the stream/string.
  ZP-1199 Utils::Utf8_truncate() needs to check that a string is valid UTF-8 even if it's shorter than the requested length.