Update files_texteditor/ajax/loadfile.php

I fixed the following bug (I put it unintentionally into core, which was not correct, sorry):

https://github.com/owncloud/core/issues/666

I have checked this bug fix with the following file types:

ANSI / ISO-8859-1
UTF-8 with BOM
UTF-8 without BOM

and it worked fine also in owncloud 4.5.5

Best wishes from Germany
Stefan Hammes