properly convert mail content to utf-8


for testing: open a mail which has a non utf8 content type - e.g. @MorrisJobke 's mail on the owncloud devel mailing list with the subject 'Re: [owncloud-devel] Is it possible to serve the minimized versions of js/css in OC7.x?'

This one has the content type:
````
Content-Type: text/plain; charset="windows-1252"; Format="flowed"
````