Fix unit tests

PHPUnit 4.3.1 by Sebastian Bergmann.

Configuration read from /home/fox/src/php/ezsystems/zetacomponents/MvcTools/phpunit.xml.dist

...............................................................  63 / 168 ( 37%)
............................................................... 126 / 168 ( 75%)
..........................................

Time: 5.78 seconds, Memory: 11.75Mb

OK (168 tests, 352 assertions)

AKAIK there is no default value for charset, (cf. src/response_writers/http.php l#205), so there should not be any charset here.