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