Add check if body is empty on 204 and 304 in specs
Kill mutant in Zero::Request::Server
Kill all mutants in Zero::Request::Accept
Kill almost all mutants in Zero::Request:AcceptType

Killed 13 of 14 mutants. I will see later, how it's possible to kill the last
one.
I also fixed the return value of "preferred", if the in initialize given string
is empty. Fixed the default value here.