setting and checking default_charset in php.ini to be UTF-8

as discussed in https://github.com/owncloud/core/issues/9764#issuecomment-50031774

@bantu @PVince81 @butonic @schiesbn please review

@karlitschek @craigpg I suggest to keep this in master for now - in case this issue pops up again in the future we could still backport this. In addition the fix in such a case is simple: set 'default_charset' to 'UTF-8' in php.ini
I have to disagree with this, we should not be overwriting users ini settings especially when we are warning them if it is not correct in the templates.

Doubly so that ini_set is disabled in most/all shared hosting enviroments.