OC5 beta 2 - text editor does not open

I think I've fixed a problem in the detection of the file encoding which prevents a text file from being opened when `mb_detect_encoding` doesn't return a value. This effect has been described here:

`http://php.net/manual/de/function.mb-detect-encoding.php`

In case that the file encoding is not recognized, the default `ISO-8859-15` is used. I don't know if an ownCloud function exists which is able to determine the systems default encoding. If yes, it would be worse to use that value as a default otherwise the given value should be ok for the beginning.

By the way this problem has already been reported, but not yet fixed, with https://github.com/owncloud/apps/issues/297