Use accessor instead of low level header value in tests

Use accessor instead of low level header value in tests

The test was failing in recent Catalyst versions because it was fetching
the header value using $res->header('Content-Type') instead of
$res->content_type. The former would get the entire HTTP header, which
recent versions would append the charset, thus changing the expected
value from 'text/x-yaml' to 'text/x-yaml; charset=UTF-8'.

The ->content_type accessor parses the header, and returns (in the
example): ('text/x-yaml', 'charset=UTF-8') in list context, and simply
'text/x-yaml' in scalar context. Using this, the test doesn't have to
worry about low level details such as the charset defined, which is out
of the scope of the test.