Change encoding in OWA from QP to UTF-8

Hello,

we changed settings as suggested here, however, mail sent via OWA or Outlook are always QP.

For our business purposes we need to have all mails encoded in UTF-8 format.

We use Exchange 2013 fully patched. 

Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable 

If anyone can suggest some solution, i would be thankfull, as at this point i don't know where to look for setting to force owa to use UTF-8.

Thanks in advance,

Ivica